Method and architecture for improving defect detectability, coupling area, and flexibility of NVSRAM cells and arrays

• US 8,964,470 B2
• Filed: 09/25/2013
• Issued: 02/24/2015
• Est. Priority Date: 09/25/2012
• Status: Expired due to Fees

• Current Assignee: Aplus Flash Technology, Inc.
• Original Assignee: Aplus Flash Technology, Inc.
• Inventors: Lee, Peter Wung
• Primary Examiner(s): MAI, SON LUU
• Application Number:
  US14/037,356
• Publication Number:
  US 20140085978A1
• Time in Patent Office: 517 Days
• Field of Search: 365/185.08
• US Class Current: 365/185.08
• CPC Class Codes: G11C 11/005 comprising combined but independently operative RAM-ROM, RAM-PROM, RAM-EPROM cells
• CPC Class Codes: G11C 14/0063 and the nonvolatile element is an EEPROM element, e.g. a floating gate or MNOS transistor
• CPC Class Codes: G11C 15/046 using non-volatile storage elements
• CPC Class Codes: G11C 16/0483 comprising cells having several storage transistors connected in series
• CPC Class Codes: H01L 21/0262 Reduction or decomposition of gaseous compounds, e.g. CVD
• CPC Class Codes: H01L 27/0207 Geometrical layout of the components, e.g. computer aided design; custom LSI, semi-custom LSI, standard cell technique
• CPC Class Codes: H10B 10/12 comprising a MOSFET load element
• CPC Class Codes: H10B 69/00 Erasable-and-programmable ROM [EPROM] devices not provided for in groups H10B41/00 - H10B63/00, e.g. ultraviolet erasable-and-programmable ROM [UVEPROM] devices

Abstract

Several preferred embodiments of 1S1F 16T NVSRAM, 1S1F 20T NVSRAM, 1S2F 22T NVSRAM, 1S2F 14T NVSRAM cells are proposed, regardless of 1-poly, 2-poly, PMOS or NOS flash cell structures. Two separate sourcelines for the paired flash Strings are also proposed for easy adding ability for the NVSRAM circuit to detect the marginally erased Vt0 and marginally programmed Vt1 of the paired flash cell. By increasing an resistance added to common SRAM power line, the pull-down current through flash Strings to grounding source line can be made much larger than the pull-up current to improve SFwrite program operation. Simple method by increasing flash cell channel length to effectively enhance coupling area is applied to secure SRAM-to-Flash store operation under self-boost-program-inhibit scheme. 1S2F architecture also provide flexibility for alternate erasing and programming during both a recall and store operation.

Claims

1. A Nonvolatile SRAM (NVSRAM) cell with marginal threshold level detection, the NVSRAM cell comprising:

a SRAM cell comprising a first inverter having a first output node and a second inverter having a second output node, the first output node and the second output node being coupled to a first word line and two bit lines respectively via a first access transistor and a second access transistor, the first inverter and the second inverter being respectively associated with a first current and a second current sharing a common power line configured to add an adjustable resistor;

a first Flash cell comprising a first string having at least a first flash transistor sandwiched by a first select transistor and a second select transistor connected in series from a first drain terminal to a first source terminal, and comprising a second string having a second flash transistor sandwiched by a third select transistor and a fourth select transistor connected in series from a second drain terminal to a second source terminal, the first select transistor and the third select transistor being commonly gated by a first select control signal, the second select transistor and the fourth select transistor being commonly gated by a second select control signal, the first flash transistor and the second flash transistor having their gates commonly coupled to a second word line signal to control a third current through the first string from the first drain terminal to the first source terminal and a fourth current through the second string from the second drain terminal to the second source terminal, the first source terminal and the second source terminal being respectively coupled to a first source line and a second source line;

wherein the first drain terminal and the second drain terminal of the first Flash cell are coupled to either the first output node or the second output node of the SRAM cell to form a differential amplifier having one paired driver device made by the first string and the second string of the first Flash cell and one paired load device made by the first inverter and the second inverter of the SRAM cell, the second word line being configured to provide one paired input of the differential amplifier for yielding one paired output respectively to the first output node and the second output node, the adjustable resistor being configured to be substantially larger than an effective resistance of either the first string or the second string for providing a greater than 3;

1 ratio between a largest one of the third current and the fourth current over a largest one of the first current and the second current when writing a first logic state associated with a combination of two threshold levels of the first flash transistor and the second flash transistor into a second logic state associated with a combination of either a VSS=0V or a low-voltage VDD level at the first output node and the second output node.

2. The NVSRAM cell of claim 1 further comprising a Bridge device including a first, second, third, and fourth LV transistor for connecting the first output node and the second output node of the SRAM cell respectively through two cross routes to the first drain terminal and the second drain terminal of the Flash cell, wherein the first and the third LV transistors are commonly gated by a FSwrite control line and the second and the fourth LV transistors are commonly gated by a SFwrite control line;

wherein the first and the second LV transistors have a first common drain node connected to the first output node of the SRAM cell;

the second and the third LV transistors have a first common source node connected to the first drain terminal of the first Flash cell;

wherein the third and the fourth LV transistors have a second common drain node connected to the second output node of the SRAM cell;

the first and the fourth LV transistors have a second common source node connected to the second drain terminal of the first Flash cell.

3. The NVSRAM cell of claim 2 wherein the adjustable resistor being configured to be substantially larger than an effective resistance of either the first string plus one of the second LV transistor and the third LV transistor in the Bridge circuit or the second string plus one of the first LV transistor and the fourth LV transistor for having a greater than 3:

1 ratio between a largest one of the third current and the fourth current over a largest one of the first current and the second current when writing a first logic state of the first Flash cell into a second logic state of the SRAM cell.

4. The NVSRAM cell of claim 1 wherein the adjustable resistor comprises a PN-paired circuit made of a PMOS transistor coupled with a NMOS transistor connected between the common power line and an on-chip VDD line, wherein the PMOS transistor is associated with a first ratio between a first channel width and a first channel length, the NMOS transistor is associated with a second ratio between a second channel width and a second channel length, the first ratio and the second ratio being configured to be substantially smaller than 1/10.

5. The NVSRAM cell of claim 1 wherein the adjustable resistor comprises a PN-paired circuit made of a PMOS transistor coupled with a NMOS transistor connected between the common power line and an on-chip VDD line, wherein the PMOS transistor is biased to be in a bare on-state with a gate-to-source voltage difference by 0.5V or smaller and the NMOS transistor is biased to be in an off-state.

6. The NVSRAM cell of claim 1 wherein the first Flash cell is subjected to a detection operation for determining the first flash transistor in the first Flash cell having a marginally-erased threshold level after an erase operation,

wherein the detection operation includes

loading the SRAM cell with a logic state “

0”

by setting the first output node at VSS=0V and the second output node at VDD,

applying 0V voltage to the second word line,

tuning the adjustable resistor to high resistance compared to an effective resistance of the second string,

coupling the first source line to 0.5V and the second source line to VSS,

determining the first flash transistor having an erased threshold level at least smaller than −

0.5V if the SRAM cell logic state “

0”

is flipped to “

1”

state due to the fourth current through the second flash transistor is greater than the third current through the first flash transistor; and

determining the first flash transistor having a marginally-erased threshold level at least equal to 0V if the SRAM cell logic state remains at “

0”

.

7. The NVSRAM cell of claim 6 wherein the detection operation further comprises increasing the voltage above 0V applied to the second word line to a final value such that the SRAM cell logic state is flipped to “

1” and

the final value corresponds to a true threshold level of the first flash transistor that is marginally erased and is just identified.

8. The NVSRAM cell of claim 6 wherein the first Flash cell is further subjected to a detection operation for determining the second flash transistor in the first Flash cell having a marginally-erased threshold level after the erase operation,

wherein the detection operation includes

loading the SRAM cell with a logic state “

1”

by setting the first output node at VDD and the second output node at VSS=0V,

applying 0V voltage to the second word line,

tuning the adjustable resistor to high resistance compared to an effective resistance of the second string,

coupling the first source line to VSS and the second source line to 0.5V,

determining the second flash transistor having an erased threshold level at least smaller than 0V if the SRAM cell logic state “

1”

is flipped to “

0”

state due to the third current through the first flash transistor is greater than the fourth current through the second flash transistor; and

determining the second flash transistor having a marginally-erased threshold level at least greater than 0V if the SRAM cell logic state remains at “

1”

.

9. The NVSRAM cell of claim 6 wherein the detection operation comprises setting the first source line and the second source line with a voltage difference greater than 0.5V.

10. The NVSRAM cell of claim 1 wherein the first Flash cell is subjected to a detection operation for determining the second flash transistor in the first Flash cell having a marginally-programmed threshold level after programming the first flash transistor and the second flash transistor in “

X”

state so that both threshold levels are supposed to be at a same designated value of 2V,

wherein the detection operation includes

loading the SRAM cell with a logic state “

0”

by setting the first output node at VSS=0V and the second output node at VDD,

applying 2V voltage to the second word line,

tuning the adjustable resistor to high resistance compared to an effective resistance of the second string,

coupling the first source line to 0.5V and the second source line to VSS,

determining the second flash transistor having a marginally-programmed threshold level at least smaller than 2V if the SRAM cell logic state “

0”

is flipped to “

1”

state due to the fourth current through the second flash transistor is greater than the third current through the first flash transistor.

11. The NVSRAM cell of claim 1 wherein the first Flash cell is subjected to a detection operation for determining the first flash transistor in the first Flash cell having a marginally-programmed threshold level after programming the first flash transistor and the second flash transistor in “

X”

state so that both threshold levels are supposed to be at a same designated value of 2V,

wherein the detection operation includes

loading the SRAM cell with a logic state “

1”

by setting the first output node at VDD and the second output node at VSS=0V,

applying 2V voltage to the second word line,

tuning the adjustable resistor to high resistance compared to an effective resistance of the second string,

coupling the first source line to VSS and the second source line to 0.5V,

determining the first flash transistor to have a marginally-programmed threshold level at least smaller than 2V if the SRAM cell logic state “

1”

is flipped to “

0”

state due to the third current through the first flash transistor is greater than the fourth current through the second flash transistor.

12. The NVSRAM cell of claim 1 wherein each of the first flash transistor and the second flash transistor is one transistor selected from a 2-poly NMOS high-voltage floating-gate transistor, a 1-poly charge-trapping type (SONOS or MONOS) high-voltage transistor, a 2-poly PMOS high-voltage floating-gate transistor, and is associated with a channel threshold level that is assigned to −

2.0V for an erased state and to +2.0V for a programmed state.

13. The NVSRAM cell of claim 1 further comprising a second Flash cell including two flash strings substantially identical to the first string and the second string in the first Flash cell, the two flash strings having a pair of top select transistors commonly gated by a third select control signal, a pair of flash transistors commonly gated by a third word line, and a pair of bottom select transistors commonly gated by a fourth select control signal.

14. The NVSRAM cell of claim 13 wherein the first Flash cell and the second Flash cell are coupled to the SRAM cell in parallel by sharing the first/second drain terminal and the first/second source terminal but having separate word line control to respective pair of flash transistors, allowing alternative erase operation and program operation.

15. The NVSRAM cell of claim 14 wherein either the first or second Flash cell is disabled by grounding the first or fourth select control signal and enabled by coupling the first or fourth select control signal to VDD, thereby allowing the SRAM cell logic data stored in the first output node and the second output node to be programmed into a paired flash transistors in the enabled one Flash cell or allowing the logic state stored in the paired flash transistors in the enabled one Flash cell to be recalled into the SRAM cell.

16. The NVSRAM cell of claim 1 wherein each of the first string and the second string includes two dummy flash transistors respectively gated by two dummy word lines so that the first/second flash transistor is respectively sandwiched between the two dummy flash transistors.

17. The NVSRAM cell of claim 16 wherein the NVSRAM cell is subjected to a flash erase operation by

setting the first select control signal to VSS=0V and the second select control signal to VDD;

setting the first source line and the second source line to VSS;

applying a high negative voltage VNN to the second word line and the two dummy word lines to erase all dummy flash transistors and the first flash transistor and the second flash transistor in the first Flash cell.

18. The NVSRAM cell of claim 16 wherein the NVSRAM cell is subjected to a programming operation from the SRAM cell to the first Flash cell by

disconnecting the common power line from the adjustable resistor;

setting the first select control signal to VSS=0V and the second select control signal to VDD;

initializing channel threshold levels of the first flash transistor and the second flash transistor by setting the two dummy word lines to VDD, the second word line to VSS, the first select control signal to VSS=0V, the second select control signal to VDD, the first and second source lines to VDD;

loading the SRAM cell with a logic state “

0”

by setting the first output node to VSS and the second output node to VDD;

changing the first select control voltage to VDD and the second word line from VDD to a positive high voltage VPP so that the first flash transistor is inhibited by a coupling voltage to maintain its initial threshold level while the second flash transistor is programmed to raise its initial threshold level at a negative value to a programmed threshold level at a positive value.

19. The NVSRAM cell of claim 16 wherein each of the first string and the second string including a flash transistor sandwiched by two dummy flash transistors between two select transistors connected in series further comprises a 5-transistor string cell layout configured to set a first channel length of the flash transistor being at least larger a combined length of drain and source N+junction areas between the flash transistor and the select transistors respectively extended by the two dummy flash transistors having a minimum pitch size, and to set a second channel length of each select transistor at 90 nm or greater.

20. The NVSRAM cell of claim 1 wherein each of the first string and the second string of the first Flash cell comprises a 3-transistor cell layout of a flash transistor between two select transistors connected in series, the cell layout being configured to set a first channel length of the flash transistor being at least larger than a combined length of drain and source N+junction areas between the flash transistor and a corresponding one select transistor, and to set a second channel length of each select transistor being selected to be 90 nm or greater.

21. A 14-transistor NVSRAM cell for flexible recall and store operations, the 14T NVSRAM cell comprising:

a SRAM cell comprising a first inverter associated with a first data node and a second inverter associated with a second data node, the first data node and the second data node being coupled to a first word line and two complementary bit lines respectively via a first access transistor and a second access transistor;

a Flash cell comprising a first string from a first drain terminal to a first source terminal and a second string from a second drain terminal to a second source terminal, the first string having a first select transistor, a first flash transistor, a second flash transistor, and a second select transistor connected in series, the second string having a third select transistor, a third flash transistor, a fourth flash transistor, and a fourth select transistor connected in series, the first flash transistor and the third flash transistor being commonly gated by a second word line signal, the second flash transistor and the fourth flash transistor being commonly gated by a third word line, the first select transistor and the third select transistor being commonly gated by a first select control signal, the second select transistor and the fourth select transistor being commonly gated by a second select control signal, the first source terminal and the second source terminal being respectively coupled to a first source line and a second source line;

wherein the first drain terminal and the second drain terminal of the Flash cell are respectively coupled to the first data node or the second data node of the SRAM cell, a first pair of the first flash transistor and the third flash transistor and a second pair of the second flash transistor and the fourth flash transistor are alternatively erased and programmed for storing either an old logic data or an updated logic data associated with a combination of two voltage levels at the first data node and the second data node of the SRAM cell during both a Store operation and a Recall operation.

22. The 14T NVSRAM cell of claim 21 wherein each of the first flash transistor the second flash transistor, the third flash transistor, and the fourth flash transistor is a 2-poly NMOS high-voltage floating-gate transistor associated with a channel threshold level that is assigned to Vt0=−

2.0V for an erased state and to Vt1=+2.0V for a programmed state.

23. The 14T NVSRAM cell of claim 22 wherein the Flash cell is subjected to an erase operation for erasing only the first pair of the first flash transistor and the third flash transistor under a bias condition of

setting the first select control signal to VSS=0V and the second select control signal to VDD;

setting the first source line and the second source line to VSS;

setting the first word line to VSS or a ‘

don'"'"'t care’

state;

setting the second word line to a negative voltage level VNN ranging from −

15 to −

18V; and

setting the third word line to be greater than Vt1 plus a 0.5V margin;

wherein the threshold levels of the first flash transistor and the third flash transistor are assigned to an erased level Vt0=−

2.0V while wherein the threshold levels of the second flash transistor and the fourth flash transistor remain unchanged.

24. The 14T NVSRAM cell of claim 22 wherein the Flash cell is subjected to an erase operation for erasing only the second pair of the second flash transistor and the fourth flash transistor under a bias condition of

setting the first select control signal to VSS=0V and the second select control signal to VDD;

setting the first source line and the second source line to VSS;

setting the first word line to VSS;

setting the second word line to either VDD or VSS; and

setting the third word line to a negative voltage level VNN ranging from −

15V to −

18V;

wherein the threshold levels of the second flash transistor and the fourth flash transistor are assigned to an erased level Vt0=−

2.0V while the threshold levels of the first flash transistor and the third flash transistor remain unchanged.

25. The 14T NVSRAM cell of claim 22 wherein the Store operation comprises a high-voltage operation for programming only the first pair of the first flash transistor and the third flash transistor under a bias condition of

setting the first select control signal to VDD and the second select control signal to VSS=0V;

setting the first source line and the second source line to VDD;

setting the first word line to VSS with a loaded SRAM logic state “

0”

by setting the first data node to VSS and the second data node to VDD;

setting the second word line to a high positive level VPP ranging from 15V to 18V;

setting the third word line to any value selected from VSS, VDD, and 0.5VPP;

wherein the first flash transistor is programmed to Vt1 level while the second flash transistor is program-inhibited to stay at Vt0 level.

26. The 14T NVSRAM cell of claim 22 wherein the Store operation comprises a high-voltage operation for programming only the second pair of the second flash transistor and the fourth flash transistor under a bias condition of

setting the first select control signal to VDD and the second select control signal to VSS=0V;

setting the first source line and the second source line to VDD;

setting the first word line to VSS with a loaded SRAM logic state “

0”

by setting the first data node to VSS and the second data node to VDD;

setting the second word line to a voltage greater than Vt1 plus a 0.5V margin;

setting the third word line to a positive level VPP ranging from 15V to 18V;

wherein the second flash transistor is programmed to Vt1 level while the fourth flash transistor is program-inhibited to stay at Vt0 level.

27. The 14T NVSRAM cell of claim 22 wherein the Recall operation comprises a low-voltage operation to load a logic state “

0”

corresponding to a threshold level vt0 assigned to the first flash transistor and Vt1 assigned to the third flash transistor of the Flash cell into the SRAM cell upon a power-on cycle under a bias condition of

setting the first select control signal and the second select control signal to VDD;

setting the first source line and the second source line to VDD;

setting the first word line to VSS=0V;

setting the second word line to VDD and the third word line to a voltage greater than Vt1 plus a 0.5V margin;

wherein the SRAM cell is set at logic state “

1”

as the first data node is set to VDD and the second data node is set to VSS due to a higher current pulling up from the first source line through the first string than that from the second source line through the second string.

28. The 14T NVSRAM cell of claim 22 wherein the Recall operation comprises a low-voltage operation to load a logic state “

0”

corresponding to a threshold level vt0 assigned to the second flash transistor and Vt1 assigned to the fourth flash transistor of the Flash cell into the SRAM cell upon a power-on cycle under a bias condition of

setting the first select control signal and the second select control signal to VDD;

setting the first source line and the second source line to VDD;

setting the first word line to VSS=0V;

setting the second word line to VDD and the third word line to a voltage greater than Vt1 plus a 0.5V margin;

wherein the SRAM cell is set at logic state “

1”

as the first data node is set to VDD and the second data node is set to VSS due to a higher current pulling-up from the first source line through the first string than that from the second source line through the second string.

29. The 14T NVSRAM cell of claim 21 wherein each of the first flash transistor the second flash transistor, the third flash transistor, and the fourth flash transistor is a 1-poly charge-trapping type (SONOS or MONOS) high-voltage transistor associated with a channel threshold level that is assigned to Vt0=−

2.0V for an erased state and to Vt1=−

2.0V for a programmed state.

30. The 14T NVSRAM cell of claim 29 wherein the Flash cell is subjected to an erase operation for erasing only the first pair of the first flash transistor and the third flash transistor under a bias condition of

setting the first select control signal to VSS=0V and the second select control signal to VDD;

setting the first source line and the second source line to VSS;

setting the first word line to VSS;

setting the second word line to a negative voltage level VNN of about −

10V; and

setting the third word line to be greater than Vt1 plus a 0.5V margin;

wherein the threshold levels of the first flash transistor and the third flash transistor are assigned to an erased level Vt0=−

2.0V while wherein the threshold levels of the second flash transistor and the fourth flash transistor remain unchanged.

31. The 14T NVSRAM cell of claim 29 wherein the Recall operation comprises a low-voltage operation to load a logic state “

0”

corresponding to a threshold level vt0 assigned to the second flash transistor and Vt1 assigned to the fourth flash transistor of the Flash cell into the SRAM cell upon a power-on cycle under a bias condition of

setting the first select control signal to VSS;

setting the first source line and the second source line to VSS=0V;

setting the first word line to VSS;

setting the second word line to a voltage smaller than −

Vt1 minus a 0.5V margin and the third word line to VSS;

wherein the SRAM cell is set at logic state “

0”

as the first data node is set to VSS and the second data node is set to VDD due to a higher current pulling-down from the first source line through the first string than that from the second source line through the second string.

32. The 14T NVSRAM cell of claim 29 wherein the Store operation comprises a high-voltage operation for programming only the first pair of the first flash transistor and the third flash transistor under a bias condition of

setting the first select control signal to VSS=0V and the second select control signal to VDD;

setting the first source line and the second source line to VDD;

setting the first word line to VSS with a loaded SRAM logic state “

0”

by setting the first data node to VSS and the second data node to VDD;

setting the second word line to a positive level VPP of about 10V;

setting the third word line to any voltage selected from VDD, VSS, and 0.5VPP;

wherein the first flash transistor is programmed to Vt1 level while the third flash transistor is program-inhibited to stay at Vt0 level.

33. The 14T NVSRAM cell of claim 21 wherein each of the first flash transistor the second flash transistor, the third flash transistor, and the fourth flash transistor is a 2-poly PMOS high-voltage floating-gate transistor associated with a channel threshold level that is assigned to Vt0=−

2.0V for an erased state and to Vt1=+2.0V for a programmed state.

34. The 14T NVSRAM cell of claim 33 wherein the Recall operation comprises a low-voltage operation to load a logic state “

0”

corresponding to a threshold level vt0 assigned to the first flash transistor and Vt1 assigned to the third flash transistor of the Flash cell into the SRAM cell upon a power-on cycle under a bias condition of

setting the first select control signal to VSS;

setting the first source line and the second source line to VSS=0V;

setting the first word line to VSS;

setting the second word line to VSS and the third word line to a voltage smaller than −

Vt1 minus a 0.5V margin;

wherein the SRAM cell is set at logic state “

0”

as the first data node is set to VSS and the second data node is set to VDD due to a higher current pulling-down from the first source line through the first string than that from the second source line through the second string.

35. The 14T NVSRAM cell of claim 33 wherein the Recall operation comprises a low-voltage operation to load a logic state “

0”

corresponding to a threshold level vt0 assigned to the second flash transistor and Vt1 assigned to the fourth flash transistor of the Flash cell into the SRAM cell upon a power-on cycle under a bias condition of

setting the first select control signal to VSS;

setting the first source line and the second source line to VSS=0V;

setting the first word line to VSS;

setting the second word line to a voltage smaller than −

Vt1 minus a 0.5V margin and the third word line to VSS;

wherein the SRAM cell is set at logic state “

0”

as the first data node is set to VSS and the second data node is set to VDD due to a higher current pulling-down from the first source line through the first string than that from the second source line through the second string.

36. The 14T NVSRAM cell of claim 33 wherein the Store operation comprises a high-voltage operation for programming only the first pair of the first flash transistor and the third flash transistor under a bias condition of

setting the first select control signal to VSS=0V and the second select control signal to VDD;

setting the first source line and the second source line to VDD;

setting the first word line to VSS with a loaded SRAM logic state “

0”

by setting the first data node to VSS and the second data node to VDD;

setting the second word line to a positive level VPP ranging from 15V to 18V;

setting the third word line to any voltage selected from VDD, VSS, and 0.5VPP;

wherein the first flash transistor is programmed to Vt1 level while the third flash transistor is program-inhibited to stay at Vt0 level.

37. The 14T NVSRAM cell of claim 33 wherein the Store operation comprises a high-voltage operation for programming only the second pair of the second flash transistor and the fourth flash transistor under a bias condition of

setting the first select control signal to VSS=0V and the second select control signal to VDD;

setting the first source line and the second source line to VDD;

setting the first word line to VSS with a loaded SRAM logic state “

0”

by setting the first data node to VSS and the second data node to VDD;

setting the third word line to a positive level VPP ranging from 15V to 18V;

setting the second word line to any voltage selected from VDD, VSS, and 0.5VPP;

wherein the second flash transistor is programmed to Vt1 level while the fourth flash transistor is program-inhibited to stay at Vt0 level.

38. A NVSRAM chip capable of flexibly detecting defect bits in the unit of bit, page, and chip, the NVSRAM chip comprising,

an NVSRAM memory array formed by MxN NVSRAM cells in a matrix of M rows and N columns, each NVSRAM cell comprising a SRAM cell coupled to a Flash cell having two flash strings corresponding to a pair of flash transistors, each row forming a page commonly associated with a SRAM word line and a VSS ground line, a SRAM Vdd power line, a pair of select gate controls, a Flash word line, and a Flash source line dividable to connect respectively to the two flash strings in each Flash cell in each NVSRAM cell, each column being coupled with a pair bit lines coupled respectively to a first output node and a second output node of each SRAM cell in each NVSRAM cell;

a SRAM X-decoder coupled with the NVSRAM memory array from X-direction to provide per each row a decoding signal to the SRAM word line and a grounding line;

a Flash X-decoder coupled with the NVSRAM memory array from X-direction to provide per each row a variable signal for the Flash word line, control signals for the pair of select gate controls, and one or two bias levels to the dividable Flash source line, and to provide an on-chip VDD power supply configured to couple with each SRAM Vdd power line via a PN-paired device as an adjustable resistor;

a Match decoder coupled to the Flash X-decoder for further providing a match signal per each row sent through the Flash source line to each NVSRAM cell;

a Y-decoder configured to provides per column a pair of decoding bits for the pair of bit lines;

a SRAM sense amplifier;

an I/O buffer device;

a CE buffer device;

an OE buffer device; and

an Address buffer;

wherein the NVSRAM memory array is subjected to a flash status verification operation configured for flexibly detecting defect bits of one or more flash transistors and screening out low marginal threshold level cells in the whole NVSRAM memory array.

39. The NVSRAM chip of claim 38 wherein the NVSRAM cell comprises,

a SRAM cell comprising a first inverter having a first output node and a second inverter having a second output node, the first output node and the second output node being coupled to the SRAM word line and the pair of bit lines respectively via a first access transistor and a second access transistor, the first inverter and the second inverter being respectively associated with a first current and a second current sharing a common SRAM Vdd power line configured to add an adjustable resistor;

a first Flash cell comprising a first string having at least a first flash transistor sandwiched by a first select transistor and a second select transistor connected in series from the first drain terminal to a first source terminal, and comprising a second string having at least a second flash transistor sandwiched by a third select transistor and a fourth select transistor connected in series from a second drain terminal to a second source terminal, the first select transistor and the third select transistor being commonly gated by a first select gate control signal, the second select transistor and the fourth select transistor being commonly gated by a second select gate control signal, the first flash transistor and the second flash transistor having their gates commonly coupled to the flash word line to control a third current through the first string from the first drain terminal to the first source terminal and a fourth current through the second string from the second drain terminal to the second source terminal, the first source terminal and the second source terminal being respectively coupled to a first source line and a second source line; and

a Bridge circuit including a first, second, third, and fourth LV transistor for connecting the first output node and the second output node of the SRAM cell respectively through two cross routes to the first drain terminal and the second drain terminal of the Flash cell, wherein the first and the third LV transistors are commonly gated by a FSwrite control line and the second and the fourth LV transistors are commonly gated by a SFwrite control line;

wherein the first and the second LV transistors have a first common drain node connected to the first output node of the SRAM cell;

the second and the third LV transistors have a first common source node connected to the first drain terminal of the first Flash cell;

wherein the third and the fourth LV transistors have a second common drain node connected to the second output node of the SRAM cell;

the first and the fourth LV transistors have a second common source node connected to the second drain terminal of the first Flash cell.

40. The NVSRAM chip of claim 39 further comprising a second Flash cell coupled with the first Flash cell in parallel sharing the SRAM cell, the Bridge circuit, and the first source line and the second source line, the second Flash cell being substantially identical to the first Flash cell and being subjected to an alternative enabling and disabling to allow just one Flash cell to be erased and programmed from the SRAM cell at any one time.

41. The NVSRAM chip of claim 39 wherein each flash transistor in the first Flash cell is one type of transistor selected from a 2-poly NMOS high-voltage floating-gate transistor, a 1-poly charge-trapping type (SONOS or MONOS) high-voltage transistor, a 2-poly PMOS high-voltage floating-gate transistor, and is associated with a channel threshold level that is assigned to Vt0=−

2.0V for an erased state and to Vt1=+2.0V for a programmed state.

42. The NVSRAM chip of claim 38 wherein the flash status verification operation including a marginal Vt0 detection for the first flash transistor and the second flash transistor in the first Flash cell in each NVSRAM cell selected from a page of the NVSRAM memory array after the first flash transistor and the second flash transistor are subjected to an erase operation, the marginal Vt0 detection is performed by

loading logic state “

0”

to the SRAM cell of each NVSRAM cell in the page by setting the corresponding first output node at VSS=0V and the corresponding second output node at VDD;

applying FSwrite control line to VDD, SFwrite control line to VSS;

applying 0V voltage to the flash word line corresponding to the selected page;

tuning the corresponding adjustable resistor to high resistance compared to an effective resistance of the second string;

coupling the corresponding first source line to 0.5V and the corresponding second source line to VSS;

determining the corresponding first flash transistor having an erased Vt0 threshold level at least smaller than −

0.5V if the SRAM cell logic state “

0”

is flipped to “

1”

state;

determining the corresponding first flash transistor having a marginally-erased Vt0 threshold level at least equal to 0V if the SRAM cell logic state remains at “

0”

;

increasing voltage applied to the flash word line to a final value such that all SRAM cells'"'"' logic states are flipped to “

1”

except one SRAM cell, thereby determining that the true marginal Vt0 threshold level of the first flash transistor associated to said one SRAM cell equals to the final value and that an Address in the selected page of the NVSRAM memory array is just identified.

43. The NVSRAM chip of claim 42 wherein the marginal Vt0 detection is further performed by

loading a logic state “

1”

to the SRAM cell of each NVSRAM cell by setting the corresponding first output node at VDD and the corresponding second output node at VSS;

applying FSwrite control line to VDD, SFwrite control line to VSS;

applying 0V voltage to the flash word line corresponding to the selected page;

tuning the corresponding adjustable resistor to high resistance compared to an effective resistance of the second string;

coupling the corresponding first source line to 0V and the corresponding second source line to 0.5V;

determining the second flash transistor having an erased Vt0 threshold level at least smaller than 0V if the SRAM cell logic state “

1”

is flipped to “

0”

state;

determining the second flash transistor having a marginally-erased Vt0 threshold level at least greater than 0V if the SRAM cell logic state remains at “

1”

;

increasing voltage applied to the flash word line to a final value such that all SRAM cells'"'"' logic states are flipped to “

0”

except one SRAM cell, thereby determining that the true marginal Vt0 threshold level of the second flash transistor associated to said one SRAM cell equals to the final value and that an Address in the selected page of the NVSRAM memory array is just identified.

44. The NVSRAM chip of claim 38 wherein the flash status verification operation including a marginal Vt1 detection for the first flash transistor and the second flash transistor in the first Flash cell in each NVSRAM cell selected from a page of the NVSRAM memory array after the first Flash cell are subjected to a programming operation to be an “

X”

state with both the first flash transistor and the second flash transistor being assigned to a same Vt1 threshold level, the marginal Vt1 detection is performed by

loading a logic state “

0”

to the SRAM cell of each NVSRAM cell in the page by setting the first output node at VSS=0V and the second output node at VDD;

applying FSwrite control line to VDD, SFwrite control line to VSS;

applying 2V voltage to the flash word line corresponding to the page,

tuning the adjustable resistor to high resistance compared to an effective resistance of the second string,

coupling the first source line to 0.5V and the second source line to VSS;

determining the second flash transistor having a marginally-programmed Vt1 threshold level at least smaller than 2V if the SRAM cell logic state “

0”

is flipped to “

1”

state;

decreasing flash word line voltage from 2V such as all SRAM cells'"'"' logic states are flipped to “

1”

except one SRAM cell, thereby determining that the true marginal Vt1 threshold level of the second flash transistor associated to said one SRAM cell equals to the final value and that an Address in the selected page of the NVSRAM memory array is just identified.

45. The NVSRAM chip of claim 44 wherein the marginal Vt1 detection is further performed by

loading a logic state “

1”

to the SRAM cell of each NVSRAM cell in the page by setting the first output node at VDD and the second output node at VSS=0V;

applying FSwrite control line to VDD, SFwrite control line to VSS;

applying 2V voltage to the second word line;

tuning the adjustable resistor to high resistance compared to an effective resistance of the second string;

coupling the first source line to VSS and the second source line to 0.5V,

determining the first flash transistor to have a marginally-programmed Vt1 threshold level at least smaller than 2V if the SRAM cell logic state “

1”

is flipped to “

0”

state;

decreasing flash word line voltage from 2V such as all SRAM cells'"'"' logic states are flipped to “

0”

except one SRAM cell, thereby determining that the true marginal Vt1 threshold level of the first flash transistor associated to said one SRAM cell equals to the final value and that an Address in the selected page of the NVSRAM memory array is just identified.

46. The NVSRAM chip of claim 38 wherein the Flash X-decoder is configured to provide at least two separate groups of Flash word lines, each group of Flash word lines including M word lines plus 2M dummy word lines corresponding to a same density of SRAM in the NVSRAM memory array.

47. The NVSRAM chip of claim 38 wherein the NVSRAM cell comprises,

a SRAM cell comprising a first inverter associated with a first data node and a second inverter associated with a second data node, the first data node and the second data node being coupled to a first word line and two complementary bit lines respectively via a first access transistor and a second access transistor;

a Flash cell comprising a first string from a first drain terminal to a first source terminal and a second string from a second drain terminal to a second source terminal, the first string having a first select transistor, a first flash transistor, a second flash transistor, and a second select transistor connected in series, the second string having a third select transistor, a third flash transistor, a fourth flash transistor, and a fourth select transistor connected in series, the first flash transistor and the third flash transistor being commonly gated by a second word line signal, the second flash transistor and the fourth flash transistor being commonly gated by a third word line, the first select transistor and the third select transistor being commonly gated by a first select control signal, the second select transistor and the fourth select transistor being commonly gated by a second select control signal, the first source terminal and the second source terminal being respectively coupled to a first source line and a second source line;

wherein each flash transistor in the Flash cell is one type of transistor selected from a 2-poly NMOS high-voltage floating-gate transistor, a 1-poly charge-trapping type (SONOS or MONOS) high-voltage transistor, a 2-poly PMOS high-voltage floating-gate transistor, and is associated with a channel threshold level that is assigned to Vt0=−

2.0V for an erased state and to Vt1=+2.0V for a programmed state.