Nonvolatile semiconductor memory and method of operating the same

1. A nonvolatile semiconductor memory including a memory cell, the memory cell comprising:

a semiconductor substrate including a pair of trenches formed on a surface thereof, the pair of trenches spaced apart in an X direction;

a first electrode formed in each trench through the intervention of a first insulating film;

a second electrode formed on the semiconductor substrate at least partially between the trenches through the intervention of a second insulating film;

a third electrode extending in the X direction on the second electrode through the intervention of a third insulating film; and

wherein in a pair of selective transistors each including the first electrode, one end of one transistor in a Y direction crossing the X direction is connected to a source region through intervention of the first insulating film, and one end of the other transistor in the Y direction is connected to a drain region through the intervention of the first insulating film.

2. A nonvolatile semiconductor memory according to claim 1 , wherein the first insulating film varies in thickness at the bottom and the side of the trench.

3. A nonvolatile semiconductor memory according to claim 1 , wherein a plurality of the memory cells are arranged in matrix,

the memory cells arranged in the X direction are connected with the third electrode lying in the X direction,

the memory cells arranged in the Y direction arc connected with the first electrode lying in the Y direction, and

the memory cells are each connected to a pair of source/drain regions via selective transistors including the first electrodes, respectively, to share the source/drain regions.

4. A nonvolatile semiconductor memory according to claim 3 , wherein two memory cells adjacent in the X direction share one of the first electrodes to establish a connection with the source region via the selective transistor comprised of the shared first electrode.

5. A nonvolatile semiconductor memory according to claim 3 , wherein all the first electrodes of the selective transistors connected to the source regions are commonly connected.

6. A nonvolatile semiconductor memory according to claim 3 , wherein all the first electrodes of the selective transistors connected to the drain regions are commonly connected.

7. A nonvolatile semiconductor memory according to claim 3 , wherein the first electrodes of the selective transistors connected to the drain regions are electrically isolated such that different voltages can be applied thereto.

8. A nonvolatile semiconductor memory according to claim 3 , wherein a device isolation region is formed between adjacent first electrodes connected to the drain regions via the selective transistors.

9. A nonvolatile semiconductor memory according to claim 3 , wherein the source regions are grounded.

10. The memory of claim 1 , wherein a bottom of at least one of the trenches is doped differently with an impurity than at least part of a sidewall of the trench, so that the bottom of the trench is more heavily doped with the impurity than is said at least part of the sidewall of the trench which may or may not be doped with the impurity.

11. A nonvolatile semiconductor memory including at least one memory cell comprising:

a semiconductor substrate including a pair of trenches formed in a surface region thereof;

selective gate electrodes, of respective selective transistors, respectively formed in the pair of trenches through the intervention of respective first insulating films;

a floating gate formed on the semiconductor substrate at least partially between the trenches through the intervention of a second insulating film;

a control gate formed on the semiconductor substrate over the floating gate through the intervention of a third insulating film; and

wherein an edge portion(s) of the floating gate overlaps one or more of the selective gate electrodes.

12. The memory of claim 11 , wherein a bottom of at least one of the trenches is doped differently with an impurity than at least part of a sidewall of the trench, so that the bottom of the trench is more heavily doped with the impurity than is said at least part of the sidewall of the trench which may or may not be doped with the impurity.

13. The memory of claim 1 , wherein the first electrode comprises a selective gate, the second electrode comprises a floating gate, and the third electrode comprises a control gate.