Semiconductor device with composite conductive features and method for preparing the same

A semiconductor device, comprising:

• a first conductive feature and a second conductive feature disposed over a pattern-dense region of a semiconductor substrate;
• a third conductive feature and a fourth conductive feature disposed over a pattern-loose region of the semiconductor substrate, wherein a distance between the first conductive feature and the second conductive feature is less than a distance between the third conductive feature and the fourth conductive feature;
• a dielectric layer covering the first conductive feature, the second conductive feature, the third conductive feature, and the fourth conductive feature, wherein the dielectric layer has a first portion between the first conductive feature and the second conductive feature and a second portion between the third conductive feature and the fourth conductive feature, and a height of the second portion is greater than a height of the first portion;
• a first spacer on a sidewall of the first conductive feature;
• a second spacer on a sidewall of the second conductive feature;
• a third spacer on a sidewall of the third conductive feature;
• a fourth spacer on a sidewall of the fourth conductive feature;
• and
• an energy removable structure disposed between the first spacer and the second spacer and covered by the first portion of the dielectric layer, wherein the energy removable structure encloses an air gap between the first conductive feature and the second conductive feature;
• wherein at least one of the first conductive feature, the second conductive feature, the third conductive feature and the fourth conductive feature is a composite feature including a protection liner and a plug in the protection liner.