Multi-Patterning to Form Vias with Straight Profiles

A method comprising:

• forming a first silicon oxide layer over a low-k dielectric layer;
• forming a carbon-containing layer over the first silicon oxide layer, wherein the carbon-containing layer comprises silicon, oxygen, and carbon;
• forming a second silicon oxide layer over the carbon-containing layer;
• forming and patterning a first photo resist over the second silicon oxide layer;
• etching the second silicon oxide layer and the carbon-containing layer using the first photo resist as a first etching mask;
• and
• ashing the first photo resist using a process gas comprising oxygen, wherein the second silicon oxide layer caps the carbon-containing layer during the ashing.

A method comprising:

• forming a first silicon oxide layer;
• forming a carbon-containing layer over the first silicon oxide layer, wherein the carbon-containing layer has a first carbon atomic percentage;
• forming a second silicon oxide layer over the carbon-containing layer;
• forming and patterning a tri-layer comprising:
• a bottom layer over and contacting the second silicon oxide layer;
• a middle layer over and contacting the bottom layer, wherein the middle layer comprises carbon, and has a second carbon atomic percentage lower than the first carbon atomic percentage;
• and
• a top layer over and contacting the middle layer;
• and
• etching the second silicon oxide layer and the carbon-containing layer using the tri-layer as an etching mask.