Novel photodefinable low-k dielectric polymers based on polybenzoxazines

Abstract

The development of spin-on photodefinable dielectrics for use in microelectronic device fabrication and packaging has traditionally focused mainly on the use of soluble polymer precursors because the desired final dielectric polymer structure is generally insoluble in any appropriate casting solvent. One major drawback to this approach however is that high temperature processes (often >300 °C) after imaging and development are generally required to convert the imaged precursor polymer into the desired low dielectric constant material. These high processing temperatures make such materials impractical for a variety of applications. The goal of the work reported in this paper was to overcome this problem by developing new low dielectric constant polymers that can be formulated into photodefinable materials and processed at lower temperatures. In this work the use of a novel hexafluoroisopropanol (HFA)-substituted diamine to synthesize novel polybenzoxazines is reported. Whereas polybenzoxazoles form a five membered ring by situating an alcohol ortho to an amide, the benzoxazine rings in the polymer backbone described in this work are formed by the dehydration of a hexafluoroalcohol (HFA) group situated ortho to the amide to form a six membered ring. Of this general class of new polybenzoxazine amide alcohol precursor polymers, it will be shown that selected polymers exhibit good solubility in developer solutions and can be formulated into photosensitive compositions by addition of a DNQ inhibitor. Polybenzoxazine film properties including dielectric constant, thermal expansion coefficient, glass transition temperature, water absorption and dissolution rate have been measured. In particular, the dielectric constant for polybenzoxazine is reported to be as low as 2.2. In contrast to polybenzoxazoles which are known to cyclize at temperatures well above 280 °C, the new polymers reported here can be cyclized at temperatures as low as 210 °C. This significantly lower thermal cyclization temperature greatly alleviates many problems with other photodefinable dielectric polymers such as thermal stress build-up during curing and also allows these materials to be integrated with a wider variety of materials that would not survive the significantly higher processing temperature required with most current polyimides and polybenzoxazoles.