Bad column handling in flash memory
In a flash memory, redundant columns are used alternatively as replacement columns for replacing bad columns or to provide additional redundancy for ECC encoding. Locations of bad columns are indicated to a soft-input ECC decoder so that data bits from bad columns are treated as having a lower reliability than data bits from other columns.
1. A multi-plane flash memory array comprising:
a first plane of flash memory cells comprising first non-redundant columns and first redundant columns, the first non-redundant columns having first bad columns, wherein a number of the first bad columns is less than or equal to a number of the first redundant columns, and wherein each of the first redundant columns stores replacement data for one of the first bad columns; and
a second plane of flash memory cells comprising second non-redundant columns and second redundant columns, the second non-redundant columns having second bad columns, wherein a number of the second bad columns is greater than a number of the second redundant columns, and wherein at least a portion of the second redundant columns stores Error Correction Code (ECC) encoded data and a remaining portion of the second redundant columns stores replacement data for a portion of the second bad columns.
2. The multi-plane flash memory array of claim 1 wherein the first plane is on a first die and the second plane is on a second die.
3. The multi-plane flash memory array of claim 2 wherein the first die and the second die are selected for the multi-plane flash memory array based on the number of the first bad columns and the number of the second bad columns.
4. The multi-plane flash memory array of claim 3 further comprising a plurality of additional dies, each of the plurality of additional dies chosen based on respective numbers of the first bad columns and the second bad columns in each of the plurality of additional dies.
5. The multi-plane flash memory array of claim 1 , wherein the first plane and the second plane are provided on a same memory die.
6. The multi-plane flash memory array of claim 5 , wherein the first plane is configured to operate in parallel with the second plane.
7. The multi-plane flash memory array of claim 1 , further comprising a plurality of memory dies, each of the plurality of memory dies having the first plane and the second plane, wherein each of the plurality of memory dies is grouped into a first group based upon the number of the first bad columns and the number of the second bad columns being greater than a predetermined threshold, and each of the plurality of memory dies is grouped into a second group based upon the number of the first bad columns and the number of the second bad columns being less than the predetermined threshold.
8. The multi-plane flash memory array of claim 7 , wherein one of the plurality of memory dies from the first group is paired for operation together with one of the plurality of memory dies from the second group to form a multi-die memory array.