Erase verify for non-volatile memory

• US 20050270836A1
• Filed: 08/05/2005
• Published: 12/08/2005
• Est. Priority Date: 08/30/2001
• Status: Active Grant

• Current Assignee: Micron Technology, Inc.
• Original Assignee: Micron Technology, Inc.
• Inventors: Chevallier, Christophe J.
• Granted Patent: US 7,123,513 B2
• Time in Patent Office: Days
• US Class Current: 365/185.30
• CPC Class Codes: G11C 16/3404 Convergence or correction of memory cell threshold voltages; Repair or recovery of overerased or overprogrammed cells
• CPC Class Codes: G11C 16/344 Arrangements for verifying correct erasure or for detecting overerased cells

Abstract

A memory device verify system determines a state of memory cells in a memory. The memory includes a memory array having a plurality of memory cells coupled to bit lines. A verify circuit is coupled to the bit lines to determine if memory cells have a erase level that is within predetermined upper and lower limits. The verify circuit can include first and second comparators. In one embodiment, the first comparator is used to compare a bit line current with an upper first reference current. The second comparator is used to compare a bit line current with a lower second reference current. The comparator circuit is not limited to reference currents, but can use reference voltages to compare to a bit line voltage. The verify circuit, therefore, eliminates the need for separate bit line leakage testing to identify over-erased memory cells.

Claims

1. A non-volatile memory comprising:

an array of non-volatile memory cells arranged in columns with bit lines;

first and second comparators for comparing a bit line voltage with first and second reference voltages to respectively produce first and second output signals;

a bit line current-to-voltage converter coupled between a selected one of the bit lines and the first and second comparators to generate the bit line voltage in response to a bit line current; and

a reference current-to-voltage converter for generating the first and second reference voltages in response to a reference, the reference current-to-voltage converter comprising;

a first resistor;

a second resistor coupled in series with the first resistor;

an activation circuit to selectively allow a current to flow through the first and second resistors; and

a reference current circuit coupled to the activation circuit for generating the current through the first and second resistors.

2. The non-volatile memory of claim 1 wherein the memory cells comprise a plurality of floating gate memory cells.

3. The non-volatile memory of claim 1 wherein each memory cell is comprised of a source region, a drain region, and a control gate.

4. The non-volatile memory of claim 3 wherein the control gate of each memory cell in a row of the array is coupled by a word line.

5. The non-volatile memory of claim 3 wherein the drain regions of each memory cell in a column are coupled by a common bit line.

6. The non-volatile memory of claim 3 wherein the source regions of each memory cell in a column are coupled by a common source line.

7. The non-volatile memory of claim 1 wherein the bit line current-to-voltage converter converts a bit line current to a bit line voltage that indicates a state of the memory cell.

8. A memory device comprising:

an array of non-volatile memory cells arranged in columns using bit lines that have a bit line current indicative of a state of a memory cell;

a reference current-to-voltage converter, for generating first and second reference voltages from a reference current, the reference current-to-voltage converter comprising;

a first resistor;

a second resistor coupled in series with the first resistor; and

an activation transistor having a source coupled to the second resistor;

an inverter circuit coupled between a gate of the activation transistor and a drain of the activation transistor; and

a reference current circuit coupled to the drain of the activation transistor for generating a current through the first and second resistors;

first and second comparators for comparing a bit line voltage with the first and second reference voltages respectively to respectively produce first and second output signals;

a bit line current-to-voltage converter coupled between a selected one of the bit lines and the first and second comparators to generate the bit line voltage in response to the bit line current; and

control circuitry to perform erase operations in response to the first and second output signals.

9. The system of claim 8 and further comprising a reference current circuit coupled to the reference current-to-voltage converter for generating the reference current.

10. The system of claim 8 wherein the current through the first and second resistors creates the first reference voltage between the first and second resistors and the second reference voltage between the second resistor and the activation circuit.

11. The system of claim 10 wherein the reference current circuit comprises a floating gate transistor programmed to conduct a specific amount of current in response to a control signal.

12. The system of claim 8 wherein the memory cells comprise a plurality of floating gate memory cells.

13. A memory system comprising:

a processor for controlling the system; and

a memory device coupled to the processor and comprising;

an array of non-volatile memory cells arranged in columns using bit lines that have a bit line current indicative of a state of a memory cell;

a reference current-to-voltage converter for generating first and second reference voltages from a reference current, the converter comprising;

a first resistor;

a second resistor coupled in series with the first resistor;

an activation circuit to selectively allow a current to flow through the first and second resistors; and

a reference current circuit coupled to the activation circuit for generating the current through the first and second resistors such that a first reference voltage is available between the first and second resistors and a second reference voltage is available between the second resistor and the activation circuit;

a bit line current-to-voltage converter coupled between a selected one of the bit lines and the first and second comparators to generate a bit line voltage in response to the bit line current;

first and second comparators for comparing the bit line voltage with the first and second reference voltages respectively to respectively produce first and second output signals; and

control circuitry to perform erase operations in response to the first and second output signals.

14. The system of claim 13 wherein the control circuitry is coupled to control lines from the processor.

15. The system of claim 13 wherein the processor generates data and address lines for communicating with the memory device.

16. The system of claim 13 wherein the memory array is comprised of flash memory cells.

17. The system of claim 13 wherein the activation circuit comprises a transistor that controls the current flow through the first and second resistors.

18. The system of claim 17 wherein the activation circuit further comprises an inverter coupled between the drain and the gate of the transistor.