Method and apparatus for reducing coupling between word lines and control gate lines in a flash memory system
A method and apparatus are disclosed for reducing the coupling that otherwise can arise between word lines and control gate lines in a flash memory system due to parasitic capacitance and parasitic resistance. The flash memory system comprises an array of flash memory cells organized into rows and columns, where each row is coupled to a word line and a control gate line.
1. A flash memory system comprising:
an array of flash memory cells, wherein the flash memory cells are organized into rows and columns and each flash memory cell comprises a word line terminal, a control gate terminal, and a floating gate;
a plurality of word lines, each of the plurality of word lines coupled to the word line terminals of the flash memory cells in a row of the array;
a plurality of control gate lines, each of the plurality of control gate lines coupled to the control gate line terminals of the flash memory cells in a row of the array; and
a decoupling circuit for coupling a control gate line for a selected row for a read operation to a first voltage source and for coupling control gate lines for unselected rows for the read operation to a second voltage source, wherein the second voltage source generates a voltage that is less than the voltage generated by the first voltage source by an amount ΔV;
wherein ΔV is approximately equal to the increase in voltage of the control gate line of the selected row that is generated due to parasitic capacitance between the control gate line for the selected row and the word line for the selected row when the word line for the selected row is driven high.
2. The flash memory system of claim 1 , wherein the decoupling circuit comprises a first switch for selectively coupling the first voltage source to selected row of a read operation and a second switch for selectively coupling the second voltage source to one or more unselected rows of a read operation.
3. The flash memory system of claim 1 , wherein the flash memory cells are split-gate flash memory cells.
4. The flash memory system of claim 2 , wherein the flash memory cells are split-gate flash memory cells.
5. A method of decoupling a control gate line from an adjacent word line in a flash memory system during a read operation, the flash memory system comprising an array of flash memory cells organized in rows and columns, a plurality of word lines wherein each of the plurality of word lines is coupled to the word line terminals of the flash memory cells in a row of the array, and a plurality of control gate lines wherein each of the plurality of control gate lines is coupled to the control gate line terminals of the flash memory cells in a row of the array, each flash memory cell comprising a word line terminal, a control gate terminal, and a floating gate, the method comprising:
coupling a control gate line for a selected row for a read operation to a first voltage source;
coupling control gate lines for unselected rows for the read operation to a second voltage source, wherein the voltage generated by the second voltage source is less than the voltage generated by the first voltage source by an amount ΔV; and
coupling a word line for the selected row for the read operation to a third voltage source;
wherein, due to parasitic capacitance between the control gate line for the selected row and the word line for the selected row, the voltage of the control gate line for the selected row increases by an amount approximately equal to ΔV after the word line for the selected row is coupled to the third voltage source.
6. The method of claim 5 , wherein the step of coupling a control gate line for a selected row for a read operation to a first voltage source comprises closing a switch between the control gate line for the selected row for a read operation and the first voltage source.
7. The method of claim 6 , wherein the step of coupling control gate lines for unselected rows for a read operation to a second voltage source comprises closing a switch between the control gate lines for the unselected rows for a read operation and the second voltage source.
8. The method of claim 5 , wherein the flash memory cells are split-gate flash memory cells.
9. The method of claim 6 , wherein the flash memory cells are split-gate flash memory cells.
10. The method of claim 7 , wherein the flash memory cells are split-gate flash memory cells.