Method for preparing semiconductor device structure with air gap structure

A method for preparing a semiconductor device structure, comprising:

• forming a first metal plug, a second metal plug, a third metal plug, and a fourth metal plug over a semiconductor substrate, wherein the first metal plug and the second metal plug are formed over a pattern-dense region of the semiconductor substrate, and the third metal plug and the fourth metal plug are over a pattern-loose region of the semiconductor substrate;
• forming an energy removable liner covering the first metal plug, the second metal plug, the third metal plug, and the fourth metal plug;
• performing an etching process to remove a portion of the energy removable layer from the substrate, while remaining an energy removable block between the first metal plug and the second metal plug in the cell region;
• forming a dielectric layer covering the energy removable block and the first metal plug, the second metal plug, the third metal plug, and the fourth metal plug;
• performing a thermal treating process to transform the energy removable layer into a first air gap structure including a first air gap enclosed by liner layer;
• wherein a first portion of the dielectric layer extends between the first metal plug and the second metal plug such that the first portion of the dielectric layer and the semiconductor substrate are separated by the first air gap while a second portion of the dielectric layer extends between the third metal plug and the fourth metal plug such that the second portion of the dielectric layer is in direct contact with the semiconductor substrate.