METHOD FOR PROGRAMMING/ERASING A NON VOLATILE MEMORY CELL DEVICE

Abstract

The method for programming/erasing a non volatile memory cell device includes at least one electric stress step to apply, to at least one active oxide layer of at least one memory cell of the device, a stress electric field able to remove at least a part of charges trapped in the active oxide layer. The method may be used for devices with floating gate type memory cells. The electric stress step may include the application, to one or more terminals of at least one memory cell, of potentials able to produce an electric field on a corresponding active oxide layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics and advantages of the programming/erasing method according to the invention will be apparent from the following description of an embodiment thereof given by way of an indicative and non limiting example with reference to the drawings.

FIGS. 1A and 1B are schematic cross-sectional views illustrating a floating gate non volatile memory cell according to the prior art.

FIGS. 2A and 2B are graphs showing a trend of characteristic parameters of the cell of FIGS. 1A and 1B.

FIGS. 3A and 3B are timing diagrams schematically showing a trend of a threshold value of non volatile memory cells, in particular belonging to a same memory sector, during a programming step and an erasing step, respectively, according to the method of the present invention.

FIGS. 4A and 4B are charts showing the mean variation of a threshold voltage of memory cells programmed/erased according to the method of the present invention.

FIGS. 5A and 5B are timing diagrams schematically showing a trend of a threshold value of single-level non volatile memory cells, in particular belonging to a same memory sector, during a programming step and an erasing step, respectively, according to a method of the present invention;

FIGS. 6A and 6B are timing diagrams schematically showing a trend of a threshold value of two-level non volatile memory cells, in particular belonging to a same memory sector, during a programming step and an erasing step, respectively, according to a method of the present invention.

FIG. 7 is a graph showing experimental results of the method according to the invention applied to a Flash type memory device.

Claims

1-29. (canceled)

30. A method for operating a non volatile memory device including a plurality of memory cells each having at least one active oxide layer, the method comprising: programming at least one memory cell;erasing the at least one memory cell; andapplying, to the at least one active oxide layer of the at least one memory cell of the non volatile memory device, an electric field to remove at least a part of electric charges trapped in the at least one active oxide layer and defining at least one electric stress phase.

31. The method according to claim 30, wherein the at least one memory cell includes at least one terminal; and wherein the at least one electric stress phase comprises applying an electric stress potential to the at least one terminal of the at least one memory cell.

32. The method according to claim 30, wherein the at least one electric stress phase provides the electric field at a value sufficient to defeat forces linking electric charges to respective traps in the active oxide layer.

33. The method according to claim 32, wherein the electric stress phase provides the electric field with at a lower value than an electric field value for generating a current in the active oxide layer.

34. The method according to claim 31, further comprising testing the memory device including at least one verification phase of a group of memory cells of the memory device for different values of potentials applied to terminals thereof when subjected to a predetermined number of repeated writing operations including programming/erasing cycles.

35. The method according to claim 34, wherein the verification phase comprises: at an end of each one of the programming/erasing cycles, a test writing phase during which the memory cells of the group are written at a same threshold value; anda retention test phase accelerated by temperature, via which a measurement is carried out of a threshold variation of the group of memory cells occurring during a period when the memory device is left at a temperature constant value without carrying out subsequent writing operations.

36. The method according to claim 35, wherein said testing comprises a reading of the group of memory cells prior to and after the electric stress phase.

37. The method according to claim 34, wherein the verification phase establishes optimal stress electric field conditions in correspondence with the threshold variation measured in the testing at a minimum value.

38. The method according to claim 34, wherein the verification phase further comprises an identification phase to identify a maximum value of the electric field based upon an absence of a current in the at least one active oxide layer.

39. The method according to claim 38, wherein the identification phase comprises: applying the electric field to a non cycled memory cell for different values of the electric stress potential;measuring a threshold variation of the memory cell occurring from prior to and after applying the electric field; andidentifying a maximum value of the electric field based upon a variation of the measured threshold.

40. The method according to claim 39, wherein the identification phase further comprises repeating application of the electric field.

41. The method according to claim 30, wherein the electric stress phase is performed based upon a writing operation of the memory cells of the memory device.

42. The method according to claim 41, wherein the electric stress phase is performed prior to the programming operation.

43. The method according to claim 41, wherein the electric stress phase is performed after the programming operation.

44. The method according to claim 41, wherein the electric stress phase is performed prior to the erasing operation.

45. The method according to claim 41, wherein the erasing operation includes an initial programming phase; and wherein the electric stress phase is performed after the initial programming step of the erasing operation.

46. The method according to claim 41, wherein the electric stress phase is performed after an erasing phase of the erasing operation.

47. The method according to claim 41, wherein the electric stress phase is performed after a reprogramming phase of the erasing operation.

48. The method according to claim 41, wherein the electric stress phase comprises applying the electric field for a duration corresponding to a duration of the writing operation.

49. The method according to claim 41, wherein the electric stress phase comprises applying the electric field in parallel on a plurality of memory cells of the memory device.

50. The method according to claim 41, wherein the electric stress phase comprises applying the electric field sequentially on groups of different memory cells of the memory device.

51. The method according to claim 31, wherein applying the electric stress potential to at least one terminal of the memory cell comprises a biasing phase to bias the at least one terminal of the memory cell between at least one of a control gate, a body, a source and a drain.

52. The method according to claim 51, wherein applying the electric stress potential includes positive and negative potentials.

53. The method according to claim 51, wherein applying the electric stress potential includes applying the electric stress potential to more than one terminal of the memory cell simultaneously.

54. The method according to claim 51, wherein applying the electric stress potential comprises applying the electric stress potential to a control gate terminal lower than a stress potential of body, source and drain terminals, maintained substantially equi-potential.

55. The method according to claim 31, wherein the electric stress phase comprises applying the stress potential via at least one rectangular pulse.

56. The method according to claim 31, wherein the electric stress phase comprises applying the stress potential via at least one ramp pulse.

57. The method according to claim 30, wherein the electric stress phase is performed on a group of memory cells.

58. The method according to claim 57, wherein the group of memory cells corresponds to a sector of the memory device.