Increasing demands for faster chip speed and reduced power consumption are
driving the semiconductor industry to develop insulating layers with lower
dielectric constants. As the dielectric constant of a material is reduced,
however, it becomes increasingly difficult to achieve the mechanical
strength required to manufacture a multilevel interconnect. A new route to
the synthesis of mesoporous silica has been demonstrated on 200 mm wafers.
Silicate precursors dissolved in supercritical CO2 are infused
into a block copolymer film. The polymer is then removed, but the resulting
porous SiO2 replicates its ordered structure, enhancing the
strength of the network. Incorporation of alkyl silicates further improves
the film properties. Post-treatment to cap residual silanol groups renders
the surface of the film hydrophobic and stabilizes it to air exposure. By
appropriate choice of the block copolymer and other process parameters, the
pore size and density can be varied and k values as low as 1.8 can be
achieved. For a film with a dielectric constant of 2.25, the pore size is ∼4
nm. The hardness and modulus are 1.1 GPa and 7.8 GPa, respectively, as
measured by nanoindentation. Four-point bend measurements yield fracture
energies of 9.8 J/m2. More importantly, the film can withstand
chemical mechanical planarization (CMP) using standard oxide polishing
conditions.