Read operation for memory with compensation for coupling based on write-erase cycles
A read operation for non-storage elements compensates for floating gate-to-floating gate coupling and effects of program-erase cycles. During programming of a word line WLn+1, the threshold voltages of previously-programmed storage elements on WLn are increased due to coupling. To compensate for the increase, during a subsequent read operation of WLn, different sets of pass voltages are applied to WLn+1 for each control gate read voltage which is applied to WLn. The pass voltages vary in each different set so that they are a function of the control gate read voltage which is applied to WLn. The pass voltages may also be a function of a number of program-erase cycles. A higher amount of compensation is provided by increasing the pass voltages as the number of program-erase cycles increases.
1. A method for operating a non-volatile storage system, comprising:
tracking a number of program-erase cycles experienced by a set of storage elements in the non-volatile storage system;
receiving a request to read a state of at least one selected storage element in the set of storage elements, the set of storage elements is in communication with a set of word lines, and the at least one selected storage element is in communication with a selected word line WLn of the set of word lines; and
in response to the read request:
reading a state of at least one unselected storage element in the set of storage elements, the at least one unselected storage element is in communication with WLn+1, an adjacent, drain-side word line of WLn;
applying different state-demarcating voltages to WLn, and applying a different set of read pass voltages to WLn+1 for each state-demarcating voltage applied to WLn, a corresponding read pass voltage in each set of read pass voltages is associated with the state of the at least one unselected storage element, and a level of the corresponding read pass voltage is a function of the number of program-erase cycles; and
determining the state of the at least one selected storage element by sensing whether the at least one selected storage element is conductive when the corresponding read pass voltage is applied to WLn+1, and when one or more of the state-demarcating voltages are applied to WLn.
2. The method of claim 1 , wherein:
the level of the corresponding read pass voltage increases with the number of program-erase cycles.
3. The method of claim 1 , wherein:
the level of the corresponding read pass voltage increases with the number of program-erase cycles at a higher rate initially when the number of program-erase cycles is within a first range of program-erase cycles, and at a lower rate subsequently when the number of program-erase cycles is within a second range of program-erase cycles.
4. The method of claim 1 , wherein:
levels of at least two of the read pass voltages in each set of read pass voltages are a function of the number of program-erase cycles.
5. The method of claim 1 , wherein:
the level of the corresponding read pass voltage is a function of the state-demarcating voltages.
6. A non-volatile storage system, comprising:
a set of storage elements;
a set of word lines in communication with the set of storage elements;
the set of storage elements includes at least one selected storage element which is in communication with a selected word line WLn of the set of word lines, and at least one unselected storage element which is in communication with WLn+1, an adjacent, drain-side word line of WLn; and
one or more control circuits in communication with the set of word lines, the one or more control circuits: (a) track a number of program-erase cycles experienced by the set of storage elements, (b) receive a request to read a state of the at least one selected storage element, and (c) in response to the read request: (i) read a state of the at least one unselected storage element, (ii) apply different state-demarcating voltages to WLn, (e) apply a different set of read pass voltages to WLn+1 for each state-demarcating voltage applied to WLn, a corresponding read pass voltage in each set of read pass voltages is associated with the state of the at least one unselected storage element, and a level of the corresponding read pass voltage is a function of the number of program-erase cycles, and (iii) determine the state of the at least one selected storage element by sensing whether the at least one selected storage element is conductive when the corresponding read pass voltage is applied to WLn+1, and when one or more of the state-demarcating voltages are applied to WLn.
7. The non-volatile storage system of claim 6 , wherein:
the level of the corresponding read pass voltage increases with the number of program-erase cycles.
8. The non-volatile storage system of claim 6 , wherein:
the level of the corresponding read pass voltage increases with the number of program-erase cycles at a higher rate initially when the number of program-erase cycles is within a first range of program-erase cycles, and at a lower rate subsequently when the number of program-erase cycles is within a second range of program-erase cycles.
9. The non-volatile storage system of claim 6 , wherein:
the corresponding read pass voltage differs in each set of read pass voltages.
10. The non-volatile storage system of claim 6 , wherein:
the level of the corresponding read pass voltage is a function of the state-demarcating voltages.
11. The non-volatile storage system of claim 6 , wherein:
the storage elements in the set of storage elements are arranged in NAND strings; and
the at least one unselected storage element is in a common NAND string with the at least one selected storage element.
12. A method for operating a non-volatile storage system, comprising:
receiving a request to read a state of at least one selected storage element in a set of storage elements, the set of storage elements is in communication with a set of word lines, and the at least one selected storage element is in communication with a selected word line WLn of the set of word lines; and
in response to the read request:
reading a state of at least one unselected storage element in the set of storage elements, the at least one unselected storage element is in communication with WLn+1, which is an adjacent, drain-side word line of WLn;
applying different state-demarcating voltages to WLn, and applying a different set of read pass voltages to WLn+1 for each state-demarcating voltage applied to WLn, a corresponding read pass voltage in each set of read pass voltages is associated with the state of the at least one unselected storage element, and the corresponding read pass voltage differs in at least two of the sets of read pass voltages; and
determining the state of the at least one selected storage element by sensing whether the at least one selected storage element is conductive when the corresponding read pass voltage is applied to WLn+1, and when one or more of the state-demarcating voltages are applied to WLn.
13. The method of claim 12 , wherein:
the corresponding read pass voltage differs in each set of read pass voltages.
14. The method of claim 12 , wherein:
a level of the corresponding read pass voltage is a function of the state-demarcating voltages.
15. The method of claim 12 , wherein:
the corresponding read pass voltage varies in magnitude inversely with the state-demarcating voltages such that the corresponding read pass voltage is higher when the state-demarcating voltage is lower, and the corresponding read pass voltage is lower when the state-demarcating voltage is higher.
16. The method of claim 12 , wherein:
the state of the at least one selected storage element is one of 2 N possible states; and
each different set of read pass voltages comprises 2 N different voltages.
17. The method of claim 12 , wherein:
the at least one unselected storage element is programmed at least in part after the at least one selected storage element is programmed, so that the at least one selected storage element experiences electromagnetic coupling from the at least one unselected storage element, and the different sets of read pass voltages provide compensation for the electromagnetic coupling, the compensation is a function of the state of the at least one unselected storage element and the state of the at least one selected storage element.
18. The method of claim 12 , further comprising:
tracking a number of program-erase cycles experienced by the set of storage elements, a level of the corresponding read pass voltage is a function of the number of program-erase cycles.
19. The method of claim 12 , wherein:
each different set of read pass voltages comprises a lowest read pass voltage which is the same in each different set of read pass voltages, and a first next higher read pass voltage which differs in each different set of read pass voltages.
20. The method of claim 19 , wherein:
each different set of read pass voltages comprises a second next higher read pass voltage which differs in each different set of read pass voltages.