ADAPTIVE VOLTAGE CONTROL FOR SRAM

• US 20090027990A1
• Filed: 07/23/2007
• Published: 01/29/2009
• Est. Priority Date: 07/23/2007
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Houston, Theodore W.
• Granted Patent: US 7,626,852 B2
• Time in Patent Office: Days
• US Class Current: 365/226
• CPC Class Codes: G11C 11/413 Auxiliary circuits, e.g. for addressing, decoding, driving, writing, sensing, timing or power reduction

Abstract

The present invention pertains to semiconductor memory devices, and particularly to a system and method for adaptively setting the operating voltages for SRAM for both Vtrip and SNM to reduce power while maintaining functionality and performance, based on modeling and characterizing a test structure. One embodiment comprises an SRAM array, a test structure that characterizes one or more parameters that are predictive of the SRAM functionality and outputs data of the parameters, a test controller that reads the parameters and identifies an operating voltage that satisfies predetermined yield criteria, and a voltage controller to set an operating voltage for the SRAM array based on the identified operating voltage. One method sets an operating voltage for an SRAM by reading test structure data of the parameters, analyzing the data to identify an operating voltage that satisfies predetermined yield criteria, and setting the operating voltage for the SRAM based on the identified operating voltage.

Claims

1. A system configured to set an operating voltage of an SRAM memory device, comprising:

an SRAM memory array;

a test structure configured to characterize one or more parameters predictive of a functionality of the SRAM memory array; and

a controller configured to set at least one operating voltage for the SRAM memory array based at least in part on the characterization of the parameters.

2. The system of claim 1, wherein the at least one operating voltage comprises a minimum operating voltage.

3. The system of claim 1, wherein the at least one operating voltage comprises a maximum operating voltage.

4. The system of claim 1, wherein the functionality of the SRAM array comprises a trip voltage (Vtrip) and a static noise margin (SNM).

5. The system of claim 1, wherein the functionality of the SRAM array comprises a static noise margin (SNM), and the one or more parameters comprise one or more of a threshold voltage Vtn and Vtp, drain current Idn and Idp, and read current (Iread).

6. The system of claim 1, wherein the at least one operating voltage comprises one or more of a minimum drain voltage Vdd_min, a maximum drain voltage Vdd_max, a minimum source voltage Vss_min, a maximum source voltage Vss_max, a retention voltage Vret, an n-well voltage VNWA, a p-well voltage VPWA, a maximum allowable voltage difference between an array voltage and a periphery voltage Vap_max, an array voltage, a bit line precharge voltage, a write driver voltage, a word line voltage, a difference between the array voltage and the bit line precharge voltage, and a difference between the array voltage and the word line driver voltage.

7. The system of claim 1, wherein the one or more parameters comprise one or more of a delay of a PMOS dominated delay chain, a delay of an NMOS dominated delay chain, and a transition voltage of an inverter.

8. The system of claim 1, wherein the SRAM memory array comprises an SRAM cell that comprises transistors and the test structure comprises replicas of the SRAM cell transistors.

9. The system of claim 1, wherein the controller is further configured to set the at least one operating voltage based upon a statistical variation of the SRAM.

10. The system of claim 1, wherein the at least one operating voltage set by the controller provides a margin for at least one of the operating parameters of the SRAM relative to a minimum or maximum acceptable value for the at least one operating parameter.

11. The system of claim 1, wherein the test structure comprises one of an inverter chain, a chain of SRAM cells arranged as inverters, a chain of series-parallel combinations of SRAM cells, a ring oscillator, a delay chain, a chain of NAND or NOR gates, a PMOS dominated delay chain, an NMOS dominated delay chain, and a chain of devices that replicate cell transistors of the SRAM memory array.

12. The system of claim 1, wherein the controller identifies the operating voltage that satisfies one or more predetermined yield criteria by:

measuring the characteristics of the one or more parameters over an expected range of operating voltages to obtain output data represented as a function of voltage,

analyzing the parameter values as a function of voltage to identify the operating voltage that satisfies one or more predetermined performance criteria.

13. The system of claim 12, wherein the one or more predetermined yield criteria comprises an acceptable distribution of at least one operating parameter of the SRAM associated with the one or more parameters as a function of voltage.

14. The system of claim 12, wherein the one or more predetermined performance criteria comprises one or more of a functionality of the test structure, a functionality of the SRAM memory array, a switching speed, a frequency, a transition response time, a delay time, retention of a data state, retention of a data state after a predetermined time period, and a voltage required for adequate operation of an SRAM.

15. The system of claim 1, wherein the test structure mimics at least one aspect of the SRAM.

16. The system of claim 15, wherein the aspect of the SRAM is an operating parameter of the SRAM.

17. The system of claim 15, wherein a transistor in the test structure is a replica of a transistor in the SRAM.

18. The system of claim 1, wherein the controller utilizes operating conditions in identifying the operating voltage for the SRAM memory array.

19. The system of claim 1, wherein the controller comprises one or more tables to correlate the one or more parameters to a voltage specification.

20. The system of claim 1, wherein the one or more parameters are measured over an expected range of operating voltages.

21. The system of claim 1, wherein the controller is configured to read the one or more parameters of the test structure by sweeping an input to the test structure and measuring a voltage at which an output of the test structure transitions.

22. The system of claim 1, wherein the controller utilizes one of temperature variation and device aging characteristics in identifying the operating voltage for the SRAM memory array.

23. The system of claim 1, further comprising one of a ROM and a fuse bank.

24. A method for setting an operating voltage for an SRAM memory device based on test structure characteristics of one or more parameters that predict a functionality of an SRAM array, comprising:

operating the test structure and reading the characteristics of the test structure; and

setting an operating voltage for the SRAM array based at least in part on the characteristics and a correlation with the functionality that satisfies one or more predetermined yield criteria of the SRAM array.

25. The method of claim 24, further comprising providing a test structure configured to characterize the one or more parameters predictive of the functionality of the SRAM array and to measure the test structure characteristics.

26. The method of claim 24, wherein the reading of the test structure characteristics comprises sweeping the input to test structure and measuring a voltage at which an output of the test structure transitions.

27. The method of claim 24, wherein the functionality of the SRAM array comprises a static noise margin (SNM), and the one or more parameters comprise one or more of a trip voltage (Vtrip), and read current (Iread).

28. The method of claim 24, wherein the operating voltage comprises one or more of a minimum drain voltage Vdd_min, a maximum drain voltage Vdd_max, a minimum source voltage Vss_min, a maximum source voltage Vss_max, a retention voltage Vret, an n-well voltage VNWA, a p-well voltage VPWA, a maximum allowable voltage difference between an array voltage and a periphery voltage Vap_max, an array voltage, a bit line precharge voltage, a write driver voltage, a word line voltage, a difference between the array voltage and the bit line precharge voltage, and a difference between the array voltage and the word line driver voltage.

29. The method of claim 24, further comprising generating the operating voltage for the SRAM array based on the identified functionality.

30. The method of claim 24, wherein analyzing the test structure characteristics comprises statistically evaluating the test structure data.

31. The method of claim 24, wherein the one or more predetermined yield criteria comprises a margin for at least one of the operating parameters of the SRAM relative to a minimum or maximum acceptable value for the at least one operating parameter.

32. The method of claim 24, wherein the operating voltage set for an SRAM memory device based on test structure characteristics is determined by a simulation program.

33. The method of claim 32, wherein the simulation program comprises establishing one or more tables and correlating the test structure characteristics to a minimum acceptable voltage which allows for variation among memory cells in the SRAM array.

34. The method of claim 24, wherein the test structure characteristics comprise parameter values associated with one of the one or more parameters measured over an expected range of operating voltages.

35. A method for setting an operating voltage for an SRAM memory device based on one or more measurable test structure characteristics that predict a functionality of an SRAM array, comprising:

modeling or simulating a relation of one or more parameters and voltages of the SRAM array to the statistical distribution of the SRAM array operational parameters (functionality) to provide predictions of the SRAM functionality as a function of the parameters and operating voltage of the SRAM array and to identify and to select from among the one or more parameters, which substantially correspond to the predictions;

determining a test structure that has measurable characteristics that can be correlated to the parameters predictive of the SRAM functionality as a function of operating voltage;

correlating the measurable characteristics of the test structure to the functionality of the SRAM as a function of operating voltage; and

determining and setting the operating voltage for the SRAM array based upon the predicted functionality of the SRAM array.

36. The method of claim 35, further comprising:

specifying a statistical model for use in the modeling or simulating of a relation of one or more parameters and voltages of the SRAM array to the statistical distribution of the SRAM array operational parameters (functionality).

37. The method of claim 36, further comprising:

designing the test structure.

38. The method of claim 37, further comprising:

modifying at least one of the statistical model or a test structure design if the selected parameters do not substantially correspond to the predictions, or if the operating voltage of the SRAM array can not be identified based on the measurable characteristics of the test structure, or if an operating voltage that gives acceptable SRAM yield can not be identified for anticipated conditions.

39. The method of claim 35, wherein executing the simulations of the statistical distribution of the SRAM array operational parameters comprises:

executing a sequence of evaluations of the selected SRAM array operational parameters using a simulator with parameter values chosen randomly for respective evaluations.

40. A method for providing an adaptive voltage control system, comprising:

simulating an operating characteristic of a circuit for a range of global parameters and voltages where a resultant distribution of the simulated operating characteristics is a result of local variation of components within the circuit;

simulating measurable characteristics of a test structure as a function of the global parameters;

correlating the distribution of the simulated operating characteristics of the circuit with the simulated measurable characteristics of the test structure; and

determining and setting an operating voltage for the circuit based upon the simulated measurable characteristics of the test structure as correlated to the operating characteristics of the circuit.

41. A system configured to set an operating voltage of an SRAM memory device, comprising:

an SRAM memory array comprising an SRAM memory cell;

a test structure configured to emulate an operating parameter of the memory cell and to characterize one or more parameters predictive of a functionality of the SRAM memory array; and

a controller configured to set the operating voltage for the SRAM memory array based on the characterization of the one or more parameters.

42. The system of claim 41 wherein the SRAM cell comprises an SRAM cell transistor and the test structure comprises a transistor that is a replica of the SRAM cell transistor.

43. The system of claim 42 wherein the test structure comprises a plurality of transistors that are each replicas of the SRAM cell transistor and the plurality of transistors are connected in parallel.

44. The system of claim 42 wherein the test structure comprises a plurality of transistors that are each replicas of the SRAM cell transistor and the plurality of transistors are connected in series.

45. The system of claim 41 wherein the characterization of the test structure provides a transition voltage wherein the transition voltage emulates the functionality of the SRAM cell.

46. The system of claim 45 wherein the functionality of the SRAM cell is the noise margin (SNM) of the cell.

47. The system of claim 45 wherein the functionality of the SRAM cell is the trip voltage (Vtrip) of the cell.

48. The system of claim 41 wherein the controller is configured to set a maximum bound on the SRAM operating voltage.

49. The system of claim 41 wherein the SRAM memory device further comprises SRAM peripheral circuitry and wherein the controller is configured to set the SRAM operating voltage for the SRAM memory array and a peripheral circuit operating voltage for the SRAM peripheral circuitry based on the characterization of the one or more parameters.

50. The system of claim 49 wherein the operating voltage for the SRAM array is different from the peripheral circuit operating voltage for the SRAM peripheral circuitry, and wherein the controller is configured to set a bound on the difference between the SRAM array voltage and the peripheral circuitry voltage.

51. A system configured to set an operating voltage of an SRAM memory device, comprising:

an SRAM memory array comprising an SRAM memory cell;

a test structure configured to characterize one or more parameters predictive of a functionality of the SRAM memory array, wherein the functionality of the SRAM array comprises a static noise margin (SNM); and

a controller configured to set the operating voltage for the SRAM memory array based on the characterization of the one or more parameters.

52. The system of claim 51 wherein the SRAM cell comprises an SRAM cell transistor and the test structure comprises a transistor that is a replica of the SRAM cell transistor.

53. The system of claim 52 wherein the test structure comprises a plurality of transistors that are each replicas of the SRAM cell transistor and the plurality of transistors are connected in parallel.

54. The system of claim 52 wherein the test structure comprises a plurality of transistors that are each replicas of the SRAM cell transistor and the plurality of transistors are connected in series.

55. The system of claim 51 wherein the characterization of the test structure provides a transition voltage wherein the transition voltage emulates the functionality of the SRAM cell.

56. The system of claim 55 wherein the functionality of the SRAM cell further comprises the trip voltage (Vtrip) of the cell.

57. The system of claim 51 wherein the controller is configured to set a maximum bound on the SRAM operating voltage.

58. The system of claim 51 wherein the SRAM memory device further comprises SRAM peripheral circuitry and wherein the controller is configured to set the SRAM operating voltage for the SRAM memory array and a peripheral circuit operating voltage for the SRAM peripheral circuitry based on the characterization of the one or more parameters.

59. The system of claim 58 wherein the operating voltage for the SRAM array is different from the peripheral circuit operating voltage for the SRAM peripheral circuitry, and wherein the controller is configured to set a bound on the difference between the SRAM array voltage and the peripheral circuitry voltage.

60. A system configured to set an array operating voltage and a peripheral circuit operating voltage of an SRAM memory device, comprising:

an SRAM memory array comprising an SRAM memory cell;

SRAM peripheral circuitry;

a test structure configured to characterize one or more parameters predictive of a functionality of the SRAM memory array; and

a controller configured to set the array operating voltage for the SRAM memory array and the peripheral circuit operating voltage for the SRAM peripheral circuitry based on the characterization of the one or more parameters.

61. The system of claim 60 wherein the SRAM cell comprises an SRAM cell transistor and the test structure comprises a transistor that is a replica of the SRAM cell transistor.

62. The system of claim 61 wherein the test structure comprises a plurality of transistors that are each replicas of the SRAM cell transistor and the plurality of transistors are connected in parallel.

63. The system of claim 61 wherein the test structure comprises a plurality of transistors that are each replicas of the SRAM cell transistor and the plurality of transistors are connected in series.

64. The system of claim 60 wherein the characterization of the test structure provides a transition voltage wherein the transition voltage emulates the functionality of the SRAM cell.

65. The system of claim 64 wherein the functionality of the SRAM cell is the noise margin (SNM) of the cell.

66. The system of claim 64 wherein the functionality of the SRAM cell is the trip voltage (Vtrip) of the cell.

67. The system of claim 60 wherein the controller is configured to set a maximum bound on one or more of the SRAM operating voltage and the peripheral circuit operating voltage.

68. The system of claim 60 wherein the array operating voltage for the SRAM array is different from the peripheral circuit operating voltage for the SRAM peripheral circuitry, and wherein the controller is configured to set a bound on the difference between the SRAM operating voltage and the peripheral circuit operating voltage.