Method for programming memory cells including transconductance degradation detection
The present invention relates to a method for programming a memory cell having a determined transconductance curve. The programming of the memory cell comprises a series of programming cycles each comprising a step of verifying the state of the memory cell. According to the present invention, the verify step comprises a first read of the memory cell with a first read voltage greater than a reference threshold voltage, and a second read of the memory cell with a second read voltage lower than or equal to the reference threshold voltage. The memory cell is considered not to be in the programmed state if first- and second-read currents flowing through the memory cell are above determined thresholds, and programming voltage pulses are applied to the memory cell while the latter is not in the programmed state. Application in particular to the programming of Flash memory cells.
1. A method for programming a memory cell comprising:
applying a first read voltage to a control gate of the memory cell having a floating gate capacitively coupled thereto;
comparing a first read current through the memory cell resulting from the first read voltage with a first threshold;
applying a second read voltage to the control gate of the memory cell if the first read current is less than the first threshold;
comparing a second read current through the memory cell resulting from the second read voltage with a second threshold; and
applying a programming voltage signal to the control gate if the second read current is greater than the second threshold to modify an amount of electric charge stored on the floating gate.
2. The method of claim 1 , wherein the programming voltage signal comprises a programming voltage pulse.
3. The method of claim 1 , further comprising repeating, until the second read current is less than the second threshold: applying a second read voltage to the control gate of the memory cell if the first read current is less than the first threshold; comparing a second read current through the memory cell resulting from the second read voltage with a second threshold; and applying a programming voltage signal to the control gate if the second read current is greater than the second threshold to modify an amount of electric charge stored on the floating gate.
4. The method of claim 3 , wherein repeating further comprises terminating the application of the programming voltage after a predetermined number of repetitions.
5. A method for identifying a degraded memory cell comprising:
applying a first read voltage to a control gate of the memory cell;
comparing a first read current through the memory cell resulting from the first read voltage with a first threshold;
applying a second read voltage to the control gate of the memory cell if the first read current is less than the first threshold;
comparing a second read current through the memory cell resulting from the second read voltage with a second threshold; and
identifying the memory cell as being degraded if the second read current is greater than the second threshold.
6. The method of claim 5 , wherein comparing further comprises comparing the first read current to a first reference current flowing through a non-degraded reference memory cell, wherein the first reference current comprises the first threshold.
7. The method of claim 5 , wherein comparing further comprises comparing the second read current to a second reference current, wherein the second reference current comprises the second threshold.
8. A memory comprising:
a memory cell;
a first read circuit configured to apply a first read voltage to the memory cell and compare a first read current through the memory cell resulting from the first read voltage with a first threshold;
a second read circuit configured to apply a second read voltage to the memory cell if the first read current is less than the first threshold and configured to compare a second read current through the memory cell resulting from the second read voltage with a second threshold; and
a programming circuit configured to apply a programming voltage signal to the memory cell if the second read current is greater than the second threshold, wherein the programming voltage signal at least partially programs the memory cell.
9. The memory of claim 8 , wherein the memory cell comprises a floating gate transistor.
10. The memory of claim 8 , wherein the second threshold is between zero nanoamperes and one thousand nanoamperes.
11. A method of programming a memory cell comprising:
applying a first read voltage greater than a reference voltage to a control gate of the memory cell;
applying a second read voltage less than first reference to the control gate of the memory cell;
comparing a first read current through the memory cell resulting from the first read voltage with a first current threshold;
comparing a second read current through the memory cell resulting from the second read voltage with a second current threshold; and
applying a programming voltage signal to the control gate of the memory cell in response to the memory cell not being in a target program state.
12. The method of claim 11 , further comprising:
repeating, until the second read current is less than the second current threshold, the steps of: applying a second read voltage less than the first reference to the control gate of the memory cell; comparing a second read current through the memory cell resulting from the second read voltage with a second current threshold; and applying a programming voltage signal to the control gate of the memory cell.
13. The method of claim 11 , further comprising:
repeating for a predetermined number of time the steps of: applying a second read voltage less than the first reference to the control gate of the memory cell; comparing a second read current through the memory cell resulting from the second read voltage with a second current threshold; and applying a programming voltage signal to the control gate of the memory cell.
14. The method of claim 11 , further comprising:
finding a first program state having a first value indicating that the memory cell is not programmed when the first read current is greater than the first current threshold and a second value indicating that the memory cell is programmed when the first read current is not greater than the first current threshold;
finding a second program state having a first value indicating that the memory cell is not programmed when the second read current is greater than the second current threshold and a second value indicating that the memory cell is programmed when the second read current is not greater than the second current threshold; and
finding a logical program state based upon both the first program state and the second program state.
15. The method of claim 14 , wherein the logical program state indicates the memory cell is not programmed when either the first program state or the second program state indicates that the memory cell is not programmed.
16. The method of claim 14 , further comprising:
storing the first and the second program states.