Structure, structure and method for providing an on-chip variable delay transmission line with fixed characteristic impedance

1. A transmission line structure, comprising:

a signal line;

a first ground return structure that causes a first delay and a first characteristic impedance in the transmission line structure; and

a second ground return structure that causes a second delay and a second characteristic impedance in the transmission line structure;

wherein the first delay is different from the second delay, and the first characteristic impedance is substantially the same as the second characteristic impedance, and

the first ground return structure provides a different inductance than the second ground return structure.

2. The structure of claim 1 , wherein the signal line, first ground return structure and second ground return structure are formed in a semiconductor structure.

3. The structure of claim 2 , wherein:

the signal line is formed in a first wiring level of the semiconductor structure,

the first ground return structure is formed in a second wiring level of the semiconductor structure, and

the second ground return structure is formed in a third level of the semiconductor structure.

4. The structure of claim 3 , wherein:

the first wiring level is different from the second wiring level, and

a portion of the first ground return structure is also formed in the first wiring level.

5. The structure of claim 4 , wherein:

the signal line is formed in the first wiring level of the semiconductor structure,

the first ground return structure is formed in the first wiring level, and

portions of the second ground return structure are formed in the first wiring level and the second wiring level of the semiconductor structure.

6. The structure of claim 2 , wherein a switch operates to ground one of the first and second ground return structures and to float the other of the second and first ground return structures, respectively.

7. The structure of claim 2 , wherein:

the first ground return structure comprises a first ground return rail and a first capacitance structure, and

the second ground return structure comprises a second ground return rail and a second capacitance structure.

8. The structure of claim 7 , wherein:

the first ground return rail is further away from the signal line than the second ground return rail, and

the first capacitance structure is closer to the signal line than the second capacitance structure.

9. The structure of claim 1 , wherein the first and second delays are delays of a signal in the signal line.

10. The structure of claim 1 , the first ground return structure provides a higher inductance than the second ground return structure.

11. The structure of claim 1 , wherein:

the first ground return structure comprises first ground return rails and first capacitance comb elements connected to and extending between the first ground return rails; and

the second ground return structure comprises second ground return rails and second capacitance comb elements connected to and extending between the second ground return rails.

12. The structure of claim 11 , wherein:

the signal line is in a first wiring level of a semiconductor structure;

the first ground return rails and the first capacitance comb elements are in a second wiring level of the semiconductor structure; and

the second ground return rails and the second capacitance comb elements are in a third wiring level of the semiconductor structure.

13. A transmission line structure, comprising:

a signal line;

a first ground return structure that causes a first delay and a first characteristic impedance in the transmission line structure; and

a second ground return structure that causes a second delay and a second characteristic impedance in the transmission line structure;

wherein the first delay is different from the second delay, and the first characteristic impedance is substantially the same as the second characteristic impedance,

wherein the first delay is about 16% different from the second delay.

14. The structure of claim 13 , wherein the first characteristic impedance is less than about 5% different from the second characteristic impedance.

15. A transmission line structure, comprising:

a signal line;

a first ground return structure that causes a first delay and a first characteristic impedance in the transmission line structure; and

a second ground return structure that causes a second delay and a second characteristic impedance in the transmission line structure;

wherein the first delay is different from the second delay, and the first characteristic impedance is substantially the same as the second characteristic impedance,

the first ground return structure comprises first ground return rails and first capacitance comb elements connected to and extending between the first ground return rails,

the second ground return structure comprises second ground return rails and second capacitance comb elements connected to and extending between the second ground return rails, and

the first ground return rails are sized and spaced further away from the signal line than are the second ground return rails resulting in the first ground return structure providing a higher inductance than the second ground return structure.

16. The structure of claim 15 , wherein the first capacitance comb elements are sized and spaced closer to the signal line than the second capacitance comb elements resulting in the first ground return structure providing a higher capacitance than the second ground return structure.