**5.4. Porous Network polymer by subtraction of porogen**

Numerous methods of introducing subtractive porosity into spin-on deposited materials exist. Conventional methods of substractive porosity involve the addition of molecular or supramolecular particles called 'porogens' with tailored thermal stability to the dielectric precursor[54]. The stability of these particles is such that they are not affected by the coating drying step, and they are removed by pyrolysis during final film sintering or cure at temperatures typically in the range from 300 to 400 °C. An example of a material for which the pore size and porosity, or the pore size and porogen load can be controlled independently. [55] However, it should be noted that the use of porogens should only be applied to dense materials having a k less than 2.5 and modulus over 5 GPa in order for the final material to satisfy the required mechanical property.

In organic materials, the SilK matrix has been the only known material to provide the thermal and mechanical properties at temperature up to 500oC for use in combination with porogens. C.E. Mohler et al. [56] reported on porous SiLK dielectric film properties such as pore volume, porosity, size distribution, and showed a 2.2 dielectric constant at 30% load of porogens.

In comparison with organic porous dielectric material, inorganic porous dielectric materials have been more rigorously investigated because of their superior mechanical properties. Representative studies have used polymethylsilsesquioxane (PMSQ) as matrix for the addition of various porogens such as the block copolymers, poly(styrerene-block-acrylic acid) [57], macromolecules of cyclodextrin [58], poly(caprolactone [59], and calix[4]arene [60].

Many of these studies with porogens have reported materials that have excellent mechanical and electrical properties, but lack in other practical aspects for application in microelectronics. When porogens are introduced into a matrix, critical problems may occur, such as thermal degradation products acting as a poison or contaminant within the matrix or interfacial adhesion problems. Therefore, use of porogens has yet to remain a difficult process for practical applications in microelectronics.
