Dynamic tuning of first read countermeasures
Techniques are provided for improving the accuracy of read operations of memory cells, where the threshold voltage (Vth) of a memory cell can shift depending on when the read operation occurs. Countermeasures are provided for a first read situation in which a memory is read after a power on event or after a long delay since a last read. Read voltages of lower or higher programmed data states are set according to a positive or negative temperature coefficient (Tco), respectively. Read voltages for error recovery can be set similarly. In another aspect, a wait period between a dummy voltage and a read voltage is a function of temperature. In another aspect, word line voltages of unselected blocks are set according to a negative Tco. In another aspect, pass voltages are set based on a Tco for each programmed data state.
1. An apparatus, comprising:
first means for applying voltages to word lines in a first block of memory cells via pass transistors of the first block, to perform an operation on the memory cells of the first block;
means for determining whether word lines in a second block of memory cells are in a coupled up state, wherein the word lines in the second block of memory cells are in the coupled up state when a time period since a last sensing operation in the second block is below a threshold; and
second means for applying a voltage to the word lines in the second block of memory cells via pass transistors of the second block during the operation, wherein control gates of the pass transistors of the first block and of the pass transistors of the second block are connected to one another, wherein the voltage applied to the word lines in the second block of memory cells is at an elevated voltage when the word lines in the second block of memory cells are in the coupled up state and at default voltage, lower than the elevated voltage, when the word lines in the second block of memory cells are not in the coupled up state.
2. The apparatus of claim 1 , wherein:
the operation comprises a read, program or erase operation; and
the first means applies the voltages to the word lines in the first block of memory cells when the first block of memory cells is selected for the read, program or erase operation, and while the second block of memory cells is unselected for a read, program or erase operation.
3. The apparatus of claim 1 , wherein:
the default voltage is independent of temperature.
4. The apparatus of claim 1 , wherein:
the second block of memory cells comprises a stack of alternating control gate layers and dielectric layers;
the control gate layers are connected to the memory cells of the second block; and
the memory cells are arranged in vertically extending memory holes in the stack.
5. A method, comprising:
performing an operation on a first block of memory cells, the performing the operation comprises applying voltages to word lines of the first block via pass transistors of the first block;
determining whether a time period since a last sensing operation in a second block is below a threshold; and
when the time period since the last sensing operation in the second block is below the threshold, applying an elevated voltage to word lines of the second block via pass transistors of the second block according to a function of a negative temperature coefficient, during the performing of the operation on the first block of memory cells.
6. The method of claim 5 , wherein:
when the time period since the last sensing operation in the second block is not below the threshold, applying a default voltage, lower than the elevated voltage, to the word lines of the second block via pass transistors of the second block, during the performing of the operation on the first block of memory cells.
7. The method of claim 5 , further comprising:
providing a control gate voltage which is common to the pass transistors of the first block and the pass transistors of the second block to concurrently provide the pass transistors of the first block and the pass transistors of the second block in a conductive state during the performing of the operation on the first block of memory cells.
8. The method of claim 5 , wherein:
the operation comprises a read, program or erase operation; and
the first block is selected for the read, program or erase operation while the second block is unselected for the read, program or erase operation.
9. An apparatus, comprising:
a first block of memory cells;
pass transistors associated with the first block of memory cells;
a second block of memory cells;
pass transistors associated with the second block of memory cells, and connected to the pass transistors associated with the first block of memory cells;
a timer which indicates an elapsed time since a last sensing operation in the second block; and
a control circuit configured to apply a voltage to the memory cells of the second block when the second block is unselected for an operation comprising a read, program or erase operation and the first block is selected for a read, program or erase operation, wherein the voltage applied to the memory cells of the second block is a function of temperature when the elapsed time is below a threshold and is a default level when the elapsed time is not below the threshold.
10. The apparatus of claim 9 , wherein:
the function of the temperature is a function of a negative temperature coefficient.
11. The apparatus of claim 1 , wherein:
the elevated voltage is set according to a negative temperature coefficient.
12. The method of claim 6 , wherein:
the default voltage is independent of the temperature.
13. The method of claim 6 , wherein:
the default voltage is a voltage applied to a source line of the second block.
14. The method of claim 5 , wherein:
the elevated voltage is an on-chip supply voltage.
15. The method of claim 5 , wherein:
when the time period since the last sensing operation in the second block is below the threshold, the word lines of the second block are in a coupled up state; and
the elevated voltage is high enough to prevent or reduce a threshold voltage downshift in the word lines of the second block.
16. The method of claim 5 , wherein:
when the time period since the last sensing operation in the second block is not below the threshold, the word lines of the second block are not in a coupled up state.
17. The apparatus of claim 9 , wherein:
the voltage applied to the memory cells of the second block when the elapsed time is below the threshold is greater than the default level.