BCB - a polymer for thinfilm applications

Thinfilm organic polymers have played a mayor role in the technologies of electronic packaging. Polymer films having low dielectric constants (less than 4) are being used to fabricate high density interconnect substrates, the basis for MCM-D (1). There are only few classes of polymers combining excellent electrical properties with high thermal and mechanical stability. Polyimide is well known for over 30 years. To reduce the processing steps forming vias photosensitive polymers were developed. These Photo-Polymers behave like photoresists eliminating approximately 75 percent of the process steps required to form the vias compared to dry etch materials. All commercially available Photo-PIs are negative acting i.e. they crosslink on exposure to ultraviolet light. The main disadvantages of Photo-PI are the high curing temperature (over 350°C) combined with a high degree of shrinkage (the photoreactive ester groups are burned out during the final cure) and the high degree of water absorption. BCB as a new class of high temperature thermosets was commercialized by the Dow Chemical Company under the tradename Cyclotene. BCB polymerizes without the evolution of by-products and has a very low degree of moisture absorption. A photosensitive prepolymer formulation was introduced in 1992 and commericiallized 1995. Now there are several formulations with different viscosities available from The Dow Chemical Company.