Copper interconnects separated by air gaps and method of making thereof

A method of making a semiconductor device, comprising:

• providing a substrate comprising a plurality of sacrificial rails separated by trenches;
• sputtering a non-conformal copper layer at a temperature above 150° C. in the trenches and over the sacrificial rails;
• heating the semiconductor device to a temperature above 300° C. to reflow the copper layer from the top of the sacrificial rails into the trenches to form copper rails in the trenches;
• removing at least a portion of the sacrificial rails after reflowing the copper layer;
• and
• depositing a non-conformal insulating layer over the copper rails such that air gaps are formed between adjacent copper rails, wherein the non-conformal insulating forms a continuous layer over the air gap;
• further comprising depositing a conformal liner on the sacrificial rails prior to sputtering the non-conformal copper layer, such that the liner is located on top of the sacrificial rails and covers sidewalls of the trenches;
• further comprising depositing a cap layer on a top surface of the copper rails in the trenches, such that the copper rails are sealed by the cap layer on their top surface, the liner on their sidewalls, and at least one of a conductive plug and an interlayer insulating layer on their bottom surface;
• further comprising:
• planarizing the cap layer to remove the cap layer and the liner from a top surface of the sacrificial rails to expose a top surface of the sacrificial rails while leaving a cap on each copper rail in the trenches;
• and
• selectively etching the sacrificial rails to remove at least an upper portion of the sacrificial rails without significantly etching the cap, the liner or the copper rails.