Erase verify for non-volatile memory

• US 7,057,935 B2
• Filed: 08/30/2001
• Issued: 06/06/2006
• Est. Priority Date: 08/30/2001
• Status: Active Grant

• Current Assignee: Micron Technology, Inc.
• Original Assignee: Micron Technology, Inc.
• Inventors: Chevallier, Christophe J.
• Primary Examiner(s): Tran, Andrew Q.
• Application Number:
  US09/943,479
• Publication Number:
  US 20030048664A1
• Time in Patent Office: 1,741 Days
• Field of Search: 365/185.33, 365/185.29, 365/185.3, 365/185.22, 365/185.21, 365/185.2, 365/185.03
• US Class Current: 365/185.22
• 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. Memory verify methods using two reference signal are also provided.

Claims

1. A non-volatile memory comprising:

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

a verify circuit selectively coupled to the bit lines to determine if the memory cells have an erase level that is within an erase level window defined by first and second reference signals, the circuit generating an indication of one of under-erasure, erasure, or over-erasure in response to a position of the erase level with the erase level window.

2. The non-volatile memory of claim 1 wherein the verify circuit comprises:

a first comparator with a reference input coupled to receive the first reference signal, and a second input coupled to the selected one of the bit lines; and

a second comparator with a reference input coupled to receive the second reference signal and a second input coupled to the selected one of the bit lines.

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

4. The non-volatile memory of claim 1 wherein the indication is a “

00”

for under-erasure, a “

10”

for erasure, and a “

11”

for over-erasure.

5. The non-volatile memory of claim 1 wherein the indication is a logical indication.

6. The non-volatile memory of claim 1 wherein the erase level window indicates a current level.

7. The non-volatile memory of claim 6 wherein the first and second reference signals are current signals.

8. A non-volatile memory comprising:

an array of non-volatile memory cells arranged in a row and column format such that the columns comprise bit lines including a selected bit line having a bit line current; and

a plurality of comparators for generating indication signals in response to a comparison of the bit line current with a plurality of reference currents each coupled to a comparator, each reference current indicating a different limit of an erase level window, each comparator generating an indication of one of under-erasure, erasure, or over-erasure in response to the comparison.

9. The non-volatile memory of claim 8 wherein the plurality of comparators comprise a first comparator that compares the bit line current to a lower limit reference current and a second comparator that compares the bit line current to an upper limit reference current.

10. The non-volatile memory of claim 9 wherein a bit line current that is less than the upper limit reference current and greater than the lower limit reference current is a current indicating erasure.