Method of making 3D integrated circuits

1. A method of making 3D integrated circuits, comprising the steps of:

(a) forming at least one trench in a first semiconductor wafer;

(b) filling the at least one trench with an insulator to form a filled trench;

(c) forming devices and back end of the line (BEOL) wiring on a first side of the first semiconductor wafer with at least one pad in the BEOL wiring aligned with the at least one filled trench;

(d) joining the first semiconductor wafer to a second semiconductor wafer having at least one landing pad aligned with the at least one filled trench in the first semiconductor wafer;

(e) etching the at least one filled trench to remove the insulator;

(f) forming an insulative spacer on the walls of the at least one trench;

(g) continuing etching the at least one trench until the at least one pad and the at least one landing pad are exposed; and

(h) filling the at least one trench with an electrical conductor to simultaneously connect the at least one pad and the at least one landing pad with the electrical conductor.

2. The method of claim 1 further comprising the step after step (b) of etching back the insulator.

3. The method of claim 1 further comprising the step after step (c) of depositing a layer of insulator over the BEOL wiring.

4. The method of claim 1 wherein the second semiconductor wafer has a BEOL wiring and the at least one landing pad is in the BEOL wiring.

5. The method of claim 4 wherein in step (d), the joining is by joining the BEOL wiring of the first semiconductor wafer to the BEOL wiring of the second semiconductor wafer.

6. The method of claim 1 further comprising the steps, between steps (d) and (e) of:

recessing the first semiconductor wafer on a second side so as to expose the at least one filled trench;

depositing a layer of nitride over the second side of the first semiconductor wafer and the exposed at least one filled trench; and

depositing polysilicon over the layer of nitride.

7. The method of claim 1 wherein step (f) comprises depositing an insulator in the trench and then etching out the insulator to leave an insulative spacer on the walls of the at least one trench.

8. The method of claim 1 wherein the trench in step (h) has a center portion, two ends and a width which is narrower in the center portion than in the two ends.

9. A method of making 3D integrated circuits, comprising the steps of:

(a) forming at least one trench in a first semiconductor wafer, the at least one trench extending from a first side of the first semiconductor wafer;

(b) filling the at least one trench with an insulator;

(c) forming devices and back end of the line (BEOL) wiring on the first side of the first semiconductor wafer with at least one pad in the BEOL wiring aligned with the at least one trench, the BEOL wiring having an exposed surface;

(d) obtaining a second semiconductor wafer having BEOL wiring, the BEOL wiring having at least one landing pad and an exposed surface;

(e) joining the exposed surface of the BEOL wiring of the first semiconductor wafer to the exposed surface of the BEOL wiring of the second semiconductor wafer such that the at least one landing pad is aligned with the at least one trench;

(f) etching the first semiconductor wafer from a second side so as to join with the at least one trench and continuing etching of the at least one trench to remove the insulator and stopping in the BEOL wiring;

(g) forming an oxide spacer on the walls of the at least one trench;

(h) continuing etching the at least one trench until the at least one pad and the at least one landing pad are exposed; and

(i) filling the at least one trench with an electrical conductor to simultaneously connect the at least one pad and the at least one landing pad with the electrical conductor.

10. The method of claim 9 further comprising the step after step (b) of etching back the insulator.

11. The method of claim 9 further comprising the step after step (c) of depositing a layer of insulator over the BEOL wiring.

12. The method of claim 9 further comprising the steps, between steps (e) and (f) of:

recessing the first semiconductor wafer on a second side so as to expose the at least one trench;

depositing a layer of nitride over the second side of the first semiconductor wafer and the exposed at least one trench; and

depositing polysilicon over the layer of nitride.

13. The method of claim 9 wherein step (g) comprises depositing an insulator in the trench and then etching out the insulator to leave an insulative spacer on the walls of the at least one trench.

14. The method of claim 9 wherein the trench in step (i) has a center portion, two ends and a width which is narrower in the center portion than in the two ends.