Middle-of-line (MOL) capacitance reduction for self-aligned contact in gate stack

1. A method of forming a semiconductor structure comprising:

forming a middle-of-line (MOL) oxide layer in the semiconductor structure, the MOL oxide layer including a plurality of gate stacks formed on a substrate;

forming a nitride layer over a silicide in the MOL oxide layer;

forming at least one self-aligned contact area (CA) element within the nitride layer;

selectively recessing the MOL oxide layer on a first side and a second side of the at least one self-aligned CA element leaving remaining portions of the MOL oxide layer contacting sides of the nitride layer and a top surface of a nitride;

selectively recessing a nitride cap of the plurality of gate stacks;

depositing an air-gap oxide layer introducing one or more air-gaps in the deposited air-gap oxide layer; and

reducing the air gap oxide layer to the at least one self-aligned CA element and the nitride layer.

2. The method of claim 1 , wherein each of the gate stacks of the plurality of gate stacks including the nitride cap formed over a metal gate and interlayer dielectric surrounding the metal gate, and the silicide is surrounded by the nitride that is formed in trenches between adjacent gate stacks of the plurality of gate stacks.

3. The method of claim 1 , wherein reducing the air gap oxide layer comprises performing chemical-mechanical planarization (CMP) on the deposited air-gap oxide layer.

4. The method of claim 2 , further comprising:

depositing metal caps to the at least one self-aligned CA element and at least one gate element, wherein the at least one gate element comprises a tungsten (W) gate element.

5. The method of claim 2 , further comprising:

masking the at least one self-aligned CA element of the semiconductor structure prior to selectively recessing the MOL oxide layer.

6. The method of claim 5 , wherein selectively recessing the nitride cap of the plurality of gate stacks comprises removing a portion of the nitride cap from a top of each of the plurality of gate stacks.

7. The method of claim 6 , wherein selectively recessing the nitride cap of the plurality of gate stacks comprises etching the nitride cap using reactive ion etching (RIE).

8. The method of claim 6 , further comprising:

removing the mask from the at least one self-aligned CA element and then performing the depositing of the air-gap oxide layer;

wherein each gate stack of the plurality of gate stacks include work function metal disposed between the interlayer dielectric and the substrate.

9. The method of claim 1 , wherein the one or more air-gaps in combination with the selective recessing of the MOL oxide layer and the air-gap oxide layer reduces capacitance of the MOL oxide layer.

10. The method of claim 1 , wherein the remaining portions of the MOL oxide layer directly contact the sides of the nitride layer and the top surface of the nitride.

11. A method comprising:

forming a middle-of-line (MOL) oxide layer in a semiconductor structure, the MOL oxide layer including a plurality of gate stacks formed on a substrate;

forming a first nitride layer over a first silicide in the MOL oxide layer;

forming a second nitride layer over a second silicide in the MOL oxide layer;

forming a first self-aligned contact area (CA) element within the first nitride layer and a second self-aligned CA element within the second nitride layer;

masking the first and the second self-aligned CA elements;

selectively recessing the MOL oxide layer on a first side and a second side of each of the first and the second self-aligned CA elements leaving remaining portions of the MOL oxide layer contacting sides of the first nitride layer, a top surface of the second nitride layer, a first nitride and a second nitride;

etching a nitride cap of each gate stack of the plurality of gate stacks;

removing the mask from the first and second self-aligned CA elements;

depositing an air-gap oxide layer introducing one or more air-gaps in the deposited air-gap oxide layer; and

reducing the air gap oxide layer to the first and the second self-aligned CA elements and the first and the second nitride layers.

12. The method of claim 11 , wherein:

the nitride cap of each of the gate stacks of the plurality of gate stacks is formed over a metal gate and interlayer dielectric surrounding the metal gate;

the first silicide is surrounded by the first nitride;

the first silicide is formed in a first trench;

the second silicide is surrounded by the second nitride;

the second nitride is formed in a second trench;

the first trench is disposed between a first pair of adjacent gate stacks of the plurality of gate stacks; and

the second trench is disposed between a second pair of adjacent gate stacks of the plurality of gate stacks.

13. The method of claim 12 , wherein reducing the air gap oxide layer comprises performing chemical-mechanical planarization (CMP) on the deposited air-gap oxide layer.

14. The method of claim 12 , further comprising:

depositing metal caps to the first and second self-aligned CA elements and the plurality of gate stacks.

15. The method of claim 14 , wherein metal of the metal gates of the plurality of gate stacks comprises tungsten (W) and each gate stack of the plurality of gate stacks include work function metal disposed between the interlayer dielectric and the substrate.

16. The method of claim 11 , wherein etching the nitride cap of each gate stack of the plurality of gate stacks comprises etching using reactive ion etching (RIE).

17. The method of claim 11 , wherein the one or more air-gaps in combination with the selective recessing of the MOL oxide layer and the air-gap oxide layer reduces capacitance of the MOL oxide layer.

18. The method of claim 11 , wherein the remaining portions of the MOL oxide layer directly contact the sides of the first nitride layer and the top surface of the second nitride layer.