A method and apparatus for curing and modifying a low k dielectric layer in an interconnect structure is disclosed. A spin-on low k dielectric layer which includes an organic silsesquioxane, polyarylether, bisbenzocyclobuene, or SiLK is spin coated on a substrate. The substrate is placed in a process chamber in a supercritical CO2 system and is treated at a temperature between 30° C. and 150° C. and at a pressure from 70 to 700 atmospheres. A co-solvent such as CF3—X or F—X is added that selectively replaces C—CH3 bonds with C—CF3 or C—F bonds. Alternatively, H2O2 is employed as co-solvent to replace a halogen in a C—Z bond where Z=F, Cl, or Br with an hydroxyl group. Two co-solvents may be combined with CO2 for more flexibility. The cured dielectric layer has improved properties that include better adhesion, lower k value, increased hardness, and a higher elastic modulus.