Semiconductor device having insulating layers with multiple material properties and method of fabricating same

A semiconductor device, comprising:

• a substrate;
• a plurality of conductive structures on the substrate, the conductive structures extending in parallel to each other in a first direction;
• and
• a first interlayer dielectric layer in a first trench and a second trench, the first and second trenches being between the conductive structures,
• wherein a width in a second direction of the first trench is less than a width in the second direction of the second trench,
• wherein the first interlayer dielectric layer includes a lower interlayer dielectric layer and an upper interlayer dielectric layer on the lower interlayer dielectric layer,
• wherein a mechanical strength of the upper interlayer dielectric layer is greater than a mechanical strength of the lower interlayer dielectric layer, and
• wherein a top surface of the upper interlayer dielectric layer in the first trench is at a level substantially same as a level of a top surface of the upper interlayer dielectric layer in the second trench.