Novel Thin Film Polymer Foaming Technique for Low and Ultra Low-k Dielectrics

As minimum device dimensions continue to shrink and on-chip device densities grow, signal distortion caused by capacitive coupling and crosstalk between the interconnections will increase. The current insulator material for on-chip applications is silicon dioxide (k= 3.9 4.2). The next generations of microchips require interlayer dielectrics with dielectric constants below 2.2 [I, 21. Although several high temperature resistant polymers have been synthesized recently [3], it seems that the ultra low-k range (k e 2.2) remains out of reach. Therefore new concepts based on porous structures have been introduced. These concepts are broadly based on two different principles: (i) thermal decomposition of a block copolymer composed of a thermally stable block and a thermally unstable one [4-71, (ii) thermal treatment of layers consisting of high T, polymers and thermally unstable components [a, 91. These techniques allow the fabrication of low and ultra low-k dielectrics. However, both techniques have several drawbacks: (i) the cost intensive preparation of specially designed block copolymers, (ii) the time-consuming heat treatment of the applied coatings for removal of the chemically labile and “volatile” component(s). In this paper we present an alternative thin film polymer foaming technique based on the following principle: (1) a polymer film is saturated with CO2 at high pressure, (2) the polymer/gas mixture is quenched into a supersaturated state by reducing the pressure and increasing the temperature, and consequently (3) nucleation and growth of gas cells dispersed throughout the polymer sample evolve rendering films with a microcellular structure. We will illustrate that our experimental method can be used to adjust the foam morphology with respect to porosity and cell size, which allows one to reduce the dielectric constant systematically. Delft University of Technology, Polymer Materials and Engineering Julianalaan 136,2628 BL Delft, The Netherlands Tel.: +3 1 15-278-6940, M.R. Wubbenhorst @ tnw.tudelft.nl