Semiconductor memory

1. A semiconductor memory, comprising:

real memory cells having transistors and disposed in a matrix shape;

dummy memory cells disposed on an outside of the real memory cells positioned on a peripheral part of the matrix and each having at least one transistor that has a same structure as a structure of the transistor of each of the real memory cells; and

first contacts coupling between two wiring layers laminated on a semiconductor substrate, disposed around each of the real and dummy memory cells, and shared by an adjacent real or dummy memory cell, wherein

number of the first contacts disposed in each of the dummy memory cells is set to be smaller than number of the first contacts disposed in each of the real memory cells.

2. The semiconductor memory according to claim 1 , wherein

a part of wires in the dummy memory cells is set to be in an open state.

3. The semiconductor memory according to claim 1 , wherein:

the first contacts in the dummy memory cells disposed in a boundary area adjacent to the real memory cells are shared by the real memory cells; and

at least one of the first contacts in the dummy memory cells disposed in a boundary area which is not adjacent to the real memory cells is lacking compared to the first contacts in each of the real memory cells.

4. The semiconductor memory according to claim 1 , further comprising:

a word line extending above a row of the real memory cells arranged in a first direction; and

a bit line extending above a column of the real memory cells arranged in a second direction orthogonal to the first direction, wherein

each of the dummy memory cells disposed under the word line and each of the dummy memory cells disposed under the bit line have different number of the first contacts formed therein.

5. The semiconductor memory according to claim 4 , wherein:

the real memory cells and the dummy memory cells each include a latch having complementary input/output nodes; and

each of the dummy memory cells disposed under the bit line includes a clip wire for coupling one of the complementary input/output nodes to a power supply line.

6. The semiconductor memory according to claim 1 , wherein

number and a structure of the transistors formed in each of the dummy memory cells are the same as number and structure of the transistors formed in each of the real memory cells.

7. The semiconductor memory according to claim 1 , wherein

all of the first contacts on a peripheral part of a dummy memory cell group disposed in a frame shape are lacking.

8. The semiconductor memory according to claim 7 , further comprising

tap cells disposed among rows of the real memory cells arranged in one direction and having second contacts for coupling a well region of the semiconductor substrate to a power supply line.

9. The semiconductor memory according to claim 1 , wherein:

each of the real memory cells and each of the dummy memory cells include diffusion layer areas that form sources and drains of the transistors; and

the dummy memory cells disposed in the frame shape are lacking the diffusion layer areas located on a peripheral side compared to the diffusion layer areas in the real memory cells.

10. The semiconductor memory according to claim 1 , further comprising:

a bit line extending above a column of the real memory cells arranged in one direction;

a self-timing memory cell disposed on one end side of the column of the real memory cells and storing a predetermined set logic value; and

a sense amplifier disposed on the other end side of the column of the real memory cells, operating in synchronization with an output timing of a data signal read from the self-timing memory cell, and amplifying the data signal read from each of the real memory cells.